The QED Manifesto
The development of mathematics toward greater precision
has led, as is well known, to the formalization of
large tracts of it, so that one can prove any theorem
using nothing but a few mechanical rules. -- K. Goedel
If civilization continues to advance, in the next two
thousand years the overwhelming novelty in human
thought will be the dominance of mathematical
understanding. -- A. N. Whitehead
Authorship and copyright information for this document may be found
at the end.
1. What Is the QED Project and Why Is It Important?
QED is the very tentative title of a project to build a computer
system that effectively represents all important mathematical
knowledge and techniques. The QED system will conform to the highest
standards of mathematical rigor, including the use of strict formality
in the internal representation of knowledge and the use of mechanical
methods to check proofs of the correctness of all entries in the
system.
The QED project will be a major scientific undertaking requiring the
cooperation and effort of hundreds of deep mathematical minds,
considerable ingenuity by many computer scientists, and broad support
and leadership from research agencies. In the interest of enlisting a
wide community of collaborators and supporters, we now mention five
reasons that the QED project should be undertaken.
First, the increase of mathematical knowledge during the last two
hundred years has made the knowledge, let alone understanding, of all
of even the most important mathematical results something beyond the
capacity of any human. For example, few mathematicians, if any, will
ever understand the entirety of the recently settled structure of
simple finite groups or the proof of the four color theorem.
Remarkably, however, the creation of mathematical logic and the
advance of computing technology have also provided the means for
building a computing system that represents all important mathematical
knowledge in an entirely rigorous and mechanically usable fashion.
The QED system we imagine will provide a means by which mathematicians
and scientists can scan the entirety of mathematical knowledge for
relevant results and, using tools of the QED system, build upon such
results with reliability and confidence but without the need for
minute comprehension of the details or even the ultimate foundations
of the parts of the system upon which they build. Note that the
approach will almost surely be an incremental one: the most important
and applicable results will likely become available before the more
obscure and purely theoretical ones are tackled, thus leading to a
useful system in the relatively near term.
Second, the development of high technology is an endeavor of fabulously
increasing mathematical complexity. The internal documentation of the next
generation of microprocessor chips may run, we have heard, to thousands of
pages. The specification of a major new industrial system, such as a
fly-by-wire airliner or an autonomous undersea mining operation, is likely to
be even an order of magnitude greater in complexity, not the least reason being
that such a system would perhaps include dozens of microprocessors. We believe
that an industrial designer will be able to take parts of the QED system and
use them to build reliable formal mathematical models of not only a new
industrial system but even the interaction of that system with a formalization
of the external world. We believe that such large mathematical models will
provide a key principle for the construction of systems substantially more
complex than those of today, with no loss but rather an increase in
reliability. As such models become increasingly complex, it will be a major
benefit to have them available in stable, rigorous, public form for use by
many. The QED system will be a key component of systems for verifying and
even synthesizing computing systems, both hardware and software.
The third motivation for the QED project is education. Nothing is more
important than mathematics education to the creation of infrastructure for
technology-based economic growth. The development of mathematical ability is
notoriously dependent upon `doing' rather than upon `being told' or
`remembering'. The QED system will provide, via such techniques as interactive
proof checking algorithms and an endless variety of mathematical results at all
levels, an opportunity for the one-on-one presenting, checking, and debugging
of mathematical technique, which it is so expensive to provide by the method of
one trained mathematician in dialogue with one student. QED can provide an
engaging and non-threatening framework for the carrying out of proofs by
students, in the same spirit as a long-standing program of Suppes at Stanford
for example. Students will be able to get a deeper understanding of
mathematics by seeing better the role that lemmas play in proofs and by seeing
which kinds of manipulations are valid in which kinds of structures. Today few
students get a grasp of mathematics at a detailed level, but via
experimentation with a computerized laboratory, that number will increase. In
fact, students can be used (eagerly, we think) to contribute to the development
of the body of definitions and proved theorems in QED. Let also us make the
observation that the relationship of QED to education may be seen in the
following broad context: with increasing technology available, governments will
look not only to cut costs of education but will increasingly turn to make
education and its delivery more cost-effective and beneficial for the state and
the individual.
Fourth, although it is not a practical motivation, nevertheless
perhaps the foremost motivation for the QED project is cultural.
Mathematics is arguably the foremost creation of the human mind. The
QED system will be an object of significant cultural character,
demonstrably and physically expressing the staggering depth and power
of mathematics. Like the great pyramids, the effort required
(especially early on) may be great; but the rewards can be even more
staggering than this effort. Mathematics is one of the most basic
things that unites all people, and helps illuminate some of the most
fundamental truths of nature, even of being itself. In the last one
hundred years, many traditional cultural values of our civilization
have taken a severe beating, and the advance of science has received
no small blame for this beating. The QED system will provide a
beautiful and compelling monument to the fundamental reality of truth.
It will thus provide some antidote to the degenerative effects of
cultural relativism and nihilism. In providing motivations for
things, one runs the danger of an infinite regression. In the end, we
take some things as inherently valuable in themselves. We believe
that the construction, use, and even contemplation of the QED system
will be one of these, over and above the practical values of such a
system. In support of this line of thought, let us cite Aristotle,
the Philosopher, the Father of Logic, `That which is proper to each
thing is by nature best and most pleasant for each thing; for man,
therefore, the life according to reason is best and pleasantest, since
reason more than anything else is man.' We speculate that this
cultural motivation may be the foremost motivation for the QED
project. Sheer aesthetic beauty is a major, perhaps the major, force
in the motivation of mathematicians, so it may be that such a
cultural, aesthetic motivation will be the key motivation inciting
mathematicians to participate.
Fifth, the QED system may help preserve mathematics from corruption.
We must remember that mathematics essentially disappeared from Western
civilization once, during the dark ages. Could it happen again? We
must also remember how unprecedented in the history of mathematics is
the clarity, even perfection, that developed in this century in regard
to the idea of formal proof, and the foundation of essentially the
entirety of known mathematics upon set theory. One can easily imagine
corrupting forces that could undermine these achievements. For
example, one might suspect that there is already a trend towards
believing some recent `theorems' in physics because they offer some
predictive power rather than that they have any meaning, much less
rigorous proof, with a possible erosion in established standards of
rigor. The QED system could offer an antidote to any such tendency.
The standard, impartial answer to the question `Has it been proved?'
could become `Has it been checked by the QED system?'. Such a
mechanical proof checker could provide answers immune to pressures of
emotion, fashion, and politics.
2. Some Objections to the Idea of the QED Project and Some Responses
The peculiarity of the evidence of mathematical
truths is that all the argument is on one side.
There are no objections, and no answer to
objections. -- J. S. Mill
Objection 1: Paradoxes, Incompatible Logics, etc. Anyone familiar
with the variety of mathematical paradoxes, controversies, and
incompatible logics of the last hundred years will realize that it is
a myth that there is certainty in mathematics. There is no
fundamentally justifiable view of mathematics which has wide support,
and no widely agreeable logic upon which such an edifice as QED could
be founded.
Reply to Objection 1: Although there are a variety of logics, there is
little doubt that one can describe all important logics within an
elementary logic, such as primitive recursive arithmetic, about which
there is no doubt, and within which one can reliably check proofs
presented in the more controversial logics. We plan to build the QED
system upon such a `root logic', as we discuss below extensively. But
the QED system is to be fundamentally unbiased as to the logics used
in proofs. Or if there is to be a bias, it is to be a bias towards
universal agreement. Proofs in all varieties of classical,
constructive, and intuitionist logic will be found rigorously
presented in the QED system -- with sharing of proofs between logics
where justified by metatheorems. For example, Goedel showed how to
map theorems in classical number theory into intuitionist number
theory, and E. Bishop showed how to develop much of modern mathematics
in a way that is simultaneously constructive and classical. A
mathematical logic may be regarded as being very much like a model of
the world -- one can often profit from using a model even if one
ultimately chooses an alternative model because it is more suited to
one's purposes. Furthermore, merely because some logic is so overly
strong as to be ultimately found inconsistent or so weak as to
ultimately fail to be able to express all that one hopes, one can
nevertheless often transfer almost all of the technique developed in
one logic to a subsequent, better logic.
Objection 2. Intellectual property problems. Such an enterprise as
QED is doomed because as soon as it is even slightly successful, it
will be so swamped by lawyers with issues of ownership, copyright,
trade secrecy, and patent law that the necessary wide cooperation of
hundreds of mathematicians, computer scientists, research agencies,
and institutions will become impossible.
Reply to Objection 2. In full cognizance of the dangers of this
objection, we put forward as a fundamental and initial principle that
the entirety of the QED system is to be in the international public
domain, so that all can freely benefit from it, and thus be inspired
to contribute to its further development.
Objection 3. Too much mathematics. Mathematics is now so large that
the hope of incorporating all of mathematics into a system is utterly
humanly impossible, especially since new mathematics is generated
faster than it can be entered into any system.
Reply to Objection 3. While it is certainly the case that we imagine
anyone being free to add, in a mechanically checked, rigorous fashion,
any sort of new mathematics to the QED system, it seems that as a
first good objective, we should pursue checking `named' theorems and
algorithms, the sort of things that are commonly taught in
universities, or cited as important in current mathematics and
applications of mathematics.
Objection 4. Mechanically checked formality is impossible. There is
no evidence that extremely hard proofs can be put into formal form in
less than some utterly ridiculous amount of work.
Reply to Objection 4. Based upon discussions with numerous workers in
automated reasoning, it is our view that using current proof-checking
technology, we can, using a variety of systems and expert users of
those systems, check mathematics at within a factor of ten, often much
better, of the time it takes a skilled mathematician to write down a
proof at the level of an advanced undergraduate textbook. QED will
support proof checking at the speeds and efficiencies of contemporary
proof-checking systems. In fact, we see one of the benefits of the
QED project as being a demonstration of the viability of
mechanically-assisted (-enforced) proof-checking.
Objection 5. If QED were feasible, it would have already been
underway several decades ago.
Reply to Objection 5. Many of the most well-known projects related to
QED were commenced in an era in which computing was exorbitantly
expensive and computer communication between geographically remote
groups was not possible. Now most secretaries have more computing
power than was available to most entire QED-related projects at their
inception, and rapid communication between most mathematics and
computer science departments through email, telnet, and ftp has become
almost universal. It also now seems unlikely that any one small
research group can, alone, make a major dent in the goal of
incorporating all of mathematics into a single system, but at the same
time technology has made widespread collaboration entirely feasible,
and the time seems ripe for a larger scale, collaborative effort. It
is also worth adding that research agencies may now be in a better
position to recognize the Babel of incompatible reasoning systems and
symbolic computation systems that have evolved from a plethora of
small projects without much attention to collaboration. Then perhaps
they can work towards encouraging collaboration, to minimize the lack
of interoperability due to diversity of theorem-statement languages,
proof languages, programming languages, computing platforms, quality,
and so on.
Objection 6. QED is too expensive.
Reply to Objection 6. While this objection requires careful study at
some point, we note that simply concentrating the efforts of some
currently-funded projects could go a long way towards getting QED off
the ground. Moreover, as noted above, students could contribute to
the project as an integrated part of their studies once the framework
is established, presumably at little or no cost. We can imagine a
number of professionals contributing as well. In particular, there is
currently a large body of tenured or retired mathematicians who have
little inclination for advanced research, and we believe that some of
these could be inspired to contribute to this project. It may be a
good idea to have a QED governing board to recognize contributions.
Objection 7. Good mathematicians will never agree to work with formal
systems because they are syntactically so constricting as to be
inconsistent with creativity.
Reply to Objection 7. The written body of formal logic rightly
repulses most mathematical readers. Whitehead and Russell's Principia
Mathematica did not establish mathematics in a notation that others
happily adopted. The traditional definition of formal logics is in a
form that no one can stand to use in practice, e.g., with function
symbols named f1, f2, f3, .... The absence of definitional principles
for almost all formal logics is an indication that from the
beginning, formal logics became something to be studied (for
properties such as completeness) rather than to be used by humans, the
practical visions of Leibniz and Frege notwithstanding. The
developers of proof checking and theorem-proving systems have done
little towards making their syntax tolerable to mathematicians. Yet,
on this matter of syntax, there is room for the greatest hope.
Although the subject of mechanical theorem-proving in general is beset
with intractable or unsolvable problems, a vastly improved
computer-human interface for mathematics is something easily within
the grasp of current computer theory and technology. The work of
Knuth on Tex and the widespread adoption of Tex by mathematicians and
mathematics journals demonstrates that it is no problem for computers
to deal with any known mathematical notation. Certainly, there is
hard work to be done on this problem, but it is also certainly within
the capacity of computer science to arrange for any rigorously
definable syntax to be something that can be conveniently entered into
computers, translated automatically into a suitable internal notation
for formal purposes, and later reproduced in a form pleasant to
humans. It is certainly feasible to arrange for the users of the QED
system to be able to shift their syntax as often as they please to any
new syntax, provided only that it is clear and unambiguous. Perhaps
the major obstacle here is simply the current scientific reward
system: precisely because new syntaxes, new parsers, and new
formatters are so easy to design, little or no credit (research,
academic, or financial) is currently available for working on this
topic. Let us add that we need take no position on the question
whether mathematicians can or should profit from the use of formal
notations in the discovery of serious, deep mathematics. The QED
system will be mainly useful in the final stages of proof reporting,
similar to writing proofs up in journals, and perhaps possibly never
in the discovery of new insights associated with deep results.
Objection 8. The QED system will be so large that it is inevitable
that there will be mistakes in its structure, and the QED system will,
therefore, be unreliable.
Reply to Objection 8. There is no doubt considerable room for error
in the construction of the QED system, as in any human enterprise. A
key motivation in Babbage's development of the computer was his
objective of producing mathematical tables that had fewer errors than
those produced by hand methods, an objective that has certainly been
achieved. It is our experience that even with the primitive proof
checking systems of today, errors made by humans are frequently found
by the use of such tools, errors that would perhaps not otherwise be
caught. The standard of success or failure of the QED project will
not be whether it helps us to reach the kingdom of perfection, an
unobtainable goal, but whether it permits us to construct proofs
substantially more accurately than we can with current hand methods.
In defense of the QED vision, let us assert that we believe that room
for error can be radically reduced by (a) expressing the full
foundation of the QED system in a few pages of mathematics and (b)
supporting the development of essentially independent implementations
for the basic checker. It goes without saying that in the development
of any particular subfield of mathematics, errors in the statements of
definitions and other axioms are possible. Agreement by experts in
each mathematical subfield that the definitions are `right' will be a
necessary part of establishing confidence that mechanically checked
theorems establish what is intended. There is no mechanical method
for guaranteeing that a logical formula says what a user intuitively
means.
Objection 9. The cooperation of mathematicians is essential to
building the QED edifice of proofs. However, because it is likely to
remain very tedious to prove theorems formally with mechanical proof
checkers for the foreeable future, mathematicians will have no
incentive to help.
Reply to Objection 9. To be developed, QED does not need to attract
the support of all or most mathematicians. If only a tenth of one
percent of mathematicians could be attracted, that will probably be
sufficient. And in compensation for the extra work currently
associated with entering formal mathematics in proof checking systems,
we can point out that some mathematicians may find the following
benefit sufficiently compensatory: in formally expressing mathematics,
one's own thoughts are often sharply clarified. One often achieves an
appreciation for subtle points in proofs that one might otherwise skim
over or skip. And the sheer joy of getting all the details of a hard
theorem `exactly right', because formalized and machine checked, is
great for many individuals. So we conjecture that enough
mathematicians will be attracted to the endeavor provided it can be
sufficiently organized to have a real chance of success.
Objection 10. The QED project represents an unreasonable diversion of
resources to the pursuit of the checking of ordinary mathematics when there is
so much profitably to be done in support of the verification of hardware and
software.
Reply to Objection 10. Current efforts in formal, mechanical hardware and
software verification are exceptionally introspective, focusing upon internal
matters such as compilers, operating systems, networks, multipliers, and
busses. From a mathematical point of view, essentially all these verifications
fall into a tiny, minor corner of elementary number theory. But eventually,
verification must reach out to consider the intended effect of computing
systems upon the external, continuous world with which they interact. If one
attempts to try to verify the use of a DSP chip for such potentially safety
critical applications as telecommunications, robot vision, speech synthesis, or
cat scanning, one immediately sees the need for such basic engineering
mathematics as Fourier transforms, not something at which existing verification
systems are yet much good. By including the rigorous development of the
mathematics used in engineering, the QED project will make a crucial
contribution to the advance of the verification of computing systems.
Objection 11. The notion that interesting mathematics can ever, in
practice, be formally checked is a fantasy. Whitehead and Russell
spent hundreds of pages to prove something as trivial as that 0 is not
1. The notion that computing systems can be verified is another
fantasy, based upon the misconception that mathematical proof can
guarantee properties of physical devices.
Reply to Objection 11. That many interesting, well-known results in
mathematics can be checked by machine is manifest to those who take
the trouble to read the literature. One can mention merely as
examples of mathematics mechanically checked from first principles:
Landau's book on the foundations of analysis, Girard's paradox,
Rolle's theorem, both Banach's and Knaster's fixed point theorems,
the mean value theorem for derivatives and integrals over Banach-space
valued functions, the fundamental counting theorem for groups, the
Schroeder-Bernstein theorem, the Picard-Lindelof theorem for the
existence of ODEs, Wilson's theorem, Fermat's little theorem, the law
of quadratic reciprocity, Ramsey's theorem, Goedel's incompleteness
theorem, and the Church-Rosser theorem. That it is possible to verify
mechanically a simple, general purpose microprocessor from the level
of gates and registers up through an application, via a verified
compiler, has been demonstrated. So there is no argument
against proof-checking or mechanical verification in principle, only
an ongoing and important engineering debate about cost-effectiveness.
The noisy verification debate is largely a comedy of misunderstanding.
In reaction to a perceived sanctimony of some verification
enthusiasts, some opponents impute to all enthusiasts grandiose claims
that complete satisfaction with a computing product can be established
by mathematical means. But any verification enthusiast ought to admit
that, at best, verification establishes a consistency between one
mathematical theory and another, e.g., between a formal specification
of intended behavior of a system and a formal representation of an
implementation, say in terms of gates and memory. Mathematical proof
can establish neither that a specification is what any user `really
wants' nor that a description of gates and memory corresponds to
physical reality. So whether the results of a computation will be
pleasing to or good for humans is something that cannot be formally
stated, much less proved.
3. Some Background, Being a Critique of Current Related Efforts
Although the root of logic is the same for all, the
`hoi polloi' live as though they have a private
understanding. -- Heraclitus
In some sense project QED is already underway, via a very diverse
collection of projects. Unfortunately, progress seems greatly slowed
by duplication of effort and by incompatibilities. If the many people
already involved in work related to QED had begun cooperation twenty-five
years ago in pursuing the construction of a single system (or
federation of subsystems) incorporating the work of hundreds of
scientists, a substantial part of the system, including at least all
of undergraduate and much of first year graduate mathematics and
computer science, could already have been incorporated into the QED
system by now. We offer as evidence the nontrivial fragments of that
body of theorems that has been successfully completed by existing
proof-checking systems.
The idea of QED is perhaps 300 years old, but one can imagine tracing
it back even 2500 years. We can agree that many groups and
individuals have made substantial progress on parts of this project,
yet we can ask the question, is there today any project underway which
can be reasonably expected to serve as the basis for QED? We believe
not, we are afraid not, though we would be delighted to join any such
project already underway. One of the reasons that we do not believe
there is any such project underway is that we think that there exist a
few basic, unsolved technical problems, which we discuss below. A
second reason is that few researchers are interested in doing the hard
work of checking proofs -- probably due to an absence of belief
that much of the entire QED edifice will ever be constructed. Another
reason is that we are familiar with many automated reasoning projects
but see very serious problems in many of them. Here are some of these
problems.
1. Too much code to be trusted. There have been a number of
automated reasoning systems that have checked many theorems of
interest, but the amount of code in some of these impressive systems
that must be correct if we are to have confidence in the proofs
produced by these systems is vastly greater than the few pages of text
that we wish to have as the foundation of QED.
2. Too strong a logic. There have been many good automated reasoning
systems that `wired in' such powerful rules of inference or such
powerful axioms that their work is suspect to many of those who might
be tempted to contribute to QED -- those of an intuitionistic or
constructivist bent.
3. Too limited a logic. Some projects have been developed upon
intuitionistic or constructive lines, but seem unlikely, so far
anyway, to support also the effective checking of theorems in
classical mathematics. We regard this `boot-strapping problem' -- how
to get, rigorously, from checking theorems in a weak logic to theorems
in a powerful classical logic, in an effective way -- to be a key
unsolved technical obstacle to QED. We discuss it further below.
4. Too unintelligible a logic. Some people have attempted to start
projects on a basis that is extremely obscure, at least when observed
by most of the community. We believe that if the initial, base, root
logic is not widely known, understood, and accepted, there will never
be much enthusiasm for QED, and hence it will never get off the
ground. It will take the cooperation of many, many people to build
the QED system.
5. Too unnatural a syntax. Just as QED must support a variety of
logics, so too must it support a variety of syntaxes, enough to make
most groups of mathematicians happy when they read theorems they are
looking for. It is unreasonable to expect mathematicians to have to
use some computer oriented or otherwise extremely simplified syntax
when concentrating on deep mathematical thoughts. Of course, a
rigorous development of the syntaxes will be essential, and it will be
a burden on human readers using the QED proof tree to `know' not only
the logical theory in which any theorem or procedure they are reading
is written but also to know the syntax being used.
6. Parochialism. There are many projects that have started over from
scratch rather than building upon the work of others, for reasons of
remoteness, ignorance of previous work, personalities, unavailability
of code due to intellectual property problems, and issues of grants
and publications. We are extremely sensitive to the fact that the
issue of credit for scientific work in a large scale project such as
this can be a main reason for the failure of the QED project. But we
can be hopeful that if a sufficient number of scientists unite in
supporting the QED project, then partial contributions to QED's
advancement will be seen in a very positive light in comparison to
efforts to start all over from scratch.
7. Too little extensibility. In 20 years there have been perhaps a
dozen major proof-checking projects, each representing an enormous
amount of activity, but which have `plataued out' or even evaporated.
It seems that when the original authors of these systems cease
actively working on their systems, the systems tend to die. Perhaps
this problem stems from the fact that insufficient analysis was given
to the basic problems of the root logic. Without a sufficient amount
of extensibility, everyone so far seems to have reached a point in
which checking new proofs is too much work to do by machine, even
though one knows that it is relatively easy for mathematicians to keep
making progress by hand. The reason, we suspect, is that
mathematicians are using some reflection principles or layers of
logics in ways not yet fully understood, or at least not implemented.
Mathematicians great contribution has been the continual
re-evaluating, re-conceptualizing, connecting, extending and, in
cases, discarding of theorems and areas. So each generation stands on
the shoulders of the giants before, as if they had always been there.
We are far from being able to represent mechanically such evolutionary
mathematical processes. Existing mathematical logics are typically as
`static' as possible, often not even permitting the addition of new
definitions! Important work in logic needs to be done to design
logics more adaptable to extension and evolution.
8. Too little heuristic search support. While it is in principle
possible to generate entries in the QED system entirely by hand, it
seems extremely likely that some sort of automated tools will be
necessary, including tools that do lots of search and use lots of
heuristics or strategies to control search. Some systems which have
completely eschewed such search and heuristic techniques might have
gotten much further in checking interesting theorems through such
techniques.
9. Too little care for rigor. It is notoriously easy to find `bugs' in
algorithms for symbolic computation. To make matters worse, these errors are
often regarded as of no significance by their authors, who plead that the
result returned is true `except on a set of measure zero', without explicitly
naming the set involved. The careful determination, nay, even proof, of
precisely which conditions under which a result is true is essential for
building the structure of mathematics so that one can depend on it. The QED
system will support the development of symbolic algebra programs in which
formal proofs of correctness of derivations are provided, along with the
precise statement of conditions under which the results are true.
10. Complete absence of inter-operability. One safe generalization
about current automated reasoning or symbolic computation systems is
that it is always somewhere between impossible and extremely difficult
to use any two of them together reliably and mechanically. It seems
almost essential to the inception of any major project in this area to
choose a logic and a syntax that is original, i.e., incompatible with
other tools. One major exception to this generalization is the base
syntax and logic for resolution systems. Here, standard problem sets
have been circulated for years. But even for such resolution systems
there is no standard syntax for entering problems involving such
fundamental mathematical constructs as induction schemas or
set-builder notation.
11. Too little attention paid to ease of use. The ease of use of
automated reasoning systems is perhaps lower than for any other type
of computing system available! In general, while anyone can use a
word processor, almost no one but an expert can use a proof checker to
check a difficult theorem. Perhaps this can be explained by the fact
that the designers of such systems have had to put so much of their
energies and attention into rigor, that they simply did not have
enough energy left for good interface design.
4. The Relationship of QED to Artificial Intelligence (AI)
and to Automated Reasoning (AR)
Project QED is largely independent of the question of the possibility
or utility of artificial intelligence or automated reasoning. To the
extent that mechanical aids of any kind can be used to help construct
(or shorten) entries in the QED system, we can be appreciative of such
aids, even if the aids use techniques that are from the realms of
artificial intelligence, assuming of course that what the aids
suggest doing is verifiably correct. A key fact is that it will not
matter, from the viewpoint of soundness, whether proofs were added to
the QED system by humans, dumb programs, smart programs or some
combination thereof. All of the QED system will be checkable by a
simple program, from first principles. The QED system will focus on
what is known in mathematics, both theorems and techniques, rather
than upon the problems of discovering new mathematics.
It is the view of some of us that many people who could have easily
contributed to project QED have been distracted away by the enticing
lure of AI or AR. It can be agreed that the grand visions of AI or AR
are much more interesting than a completed QED system while still
believing that there is great aesthetic, philosophical, scientific,
educational, and technological value in the construction of the QED
system, regardless of whether its construction is or is not largely
done `by hand' or largely automatically.
5. The Root Logic -- Some Technical Details
Method consists entirely in the order and disposition of
the objects towards which our mental vision must be
directed if we would find out any truth. We shall comply
with it exactly if we reduce involved and obscure
propositions step by step to those that are simpler, and
then starting with the intuitive apprehension of all those
that are absolutely simple, attempt to ascend to the
knowledge of all others by precisely similar steps.
-- R. Descartes
An important early technical step will be to `get off the ground',
logically speaking, which we will do by rooting the QED system in a
`root logic', whose description requires only a few pages of typical
logico-mathematical text. As a model for brevity and clarity, we can
refer the reader to Goedel's presentation, in about two pages, of
high-order logic with number theory and set theory, at the beginning
of his famous paper on undecidable questions.
The reason that we emphasize succinctness in the description of the
logic is that we hope that there will be many separate implementations
of a proof checker for this `root logic' and that each of these
implementations can check the correctness of the entire QED system.
In the end, it will be the `social process' of mathematical agreement
that will lead to confidence in the implementations of these
proof-checkers for the root logic of the QED system, and multiple
implementations of a succinct logic will greatly increase the chance
this social process will occur.
It is crucial that a `root logic' be a logic that is agreeable to all
practicing mathematicians. The logic will, by necessity, be
sufficiently strong to check any explicit computation, but the logic
surely must not prejudge any historically debated questions such as
the law of the excluded middle or the existence of uncountable sets.
As just one hint of a logic that might be used as the basis of QED, we
mention Primitive Recursive Arithmetic (PRA) which is the logic Skolem
invented for the foundations of arithmetic, which was later adopted by
Hilbert-Bernays as the right vehicle for proof theory. It has also
been further developed by Goodstein. In PRA one finds (a) an absence
of explicit quantification, (b) an ability to define primitive
recursive functions, (c) a few rules for handling equality, e.g.,
substitution of equals for equals, (d) a rule of instantiation, and
(e) a simple induction principle. One reason for taking such a logic
as the root logic is that it is doubtful that Metamathematics can be
developed in a weaker logic. In any root logic one needs to be able
to define, inductively, an infinite collection of terms and,
inductively, an infinite collection of theorems, using in the
definition of `theorem' such primitive recursive concepts as
substitution. Thus PRA has the bare minimum power we would need to
`get off the ground'. Yet we think it suffices even for checking
theorems in classical set theory, in a sense we describe below. The
logic FS0, conservative over PRA, but with sets and quantifiers, has
been proposed by Feferman as a vehicle more congenial than PRA for
studying logics.
It is probably the case that the syntax of resolution theorem-proving
is the most widely used and most easily understood logic in the
history of work on mechanical theorem-proving and proof checking, and
thus perhaps a resolution-like logic could serve as a natural choice
for a root logic. Some may object on the grounds that resolution,
being based upon classical first order logic, "wires in" the law of
the excluded middle, and therefore is objectionable to
constructivists. In response to this objection, let us note that
constructivists do not object to the law of the excluded middle in a
free variable setting if all of the predicates and function symbols
"in sight" are recursively defined; for example, it is a constructive
theorem that for all positive integers x and y, x divides y or x does
not divide y. Thus we might imagine taking as a root logic resolution
restricted to axioms describing recursive functions and hereditarily
finite objects, such as the integers.
The lambda-calculus-based "logical frameworks" work in Europe, in the
de Bruijn tradition, is perhaps the most well developed potential root
logic, with several substantial computer implementations which have
already checked significant parts of mathematics. And already, many
different logics have been represented in these logical frameworks.
As a caution, we note that some may worry there is dangerously too
much logical power in some of these versions of the typed lambda
calculus. But such logical frameworks give rise to the hope that the
root logic might be such that classical logic could simply be viewed
as the extension of the root logic by a few higher-order axioms such
as (all P) (Or P (Not P)).
One possible argument in favor of adopting a root logic of power PRA
is that its inductive power permits the proof of metatheorems, which
will enable the QED system to check and then effectively use decision
procedures. For example, the deduction theorem for first order
logic is a theorem of FS0, something not provable in some logical
framework systems, for want of induction.
Regardless of the strength or weakness of the root logic chosen, we
believe that we can rigorously incorporate into the QED system any
part of mathematics, including extremely non-constructive set
theoretic arguments, because we can represent these arguments `one
level removed' as `theorems' that a certain finite object is indeed a
proof in a certain theory. For example, if we have in mind some high
powered theorem, say, the independence of the continuum hypothesis, we
can immediately think of a corresponding theorem of primitive
recursive arithmetic that says, roughly, that some sequence of
formulas is a proof in some suitable set theory, S1, of another
theorem about some other set theory, where a, say, primitive recursive
proof checker for S1 has been written in the root logic of QED. In
practice, it will be highly advantageous if we make it appear that one
isn't really proving a theorem of proof theory but rather is proving a
theorem of group theory or topology or whatever.
Although many groups have built remarkable theorem-proving and proof
checking systems, we believe that there is a need for some further
scientific or computational advances to overcome some `resource'
problems in building a system that can hold all important mathematics.
Simply stated, it appears that complete proofs of certain theorems
that involve a lot of computation will require more disk space for
their storage than could reasonably be expected to be available for the
project. The most attractive solution to such a problem is the
development of `reflection' techniques that will permit one to use
algorithms that have been rigorously incorporated within QED as part
of the QED proof system.
Although we have spoken of a single root logic, we need to make clear
that we do not want to fall into the trap of searching for a single,
ideal logic. We can easily imagine that it will be possible to
develop several different root logics each of which can be fully
regarded to be `a' foundation of QED, each of which is capable as
acting as a basis for the other, and each of which has very short
implementations which have been checked by the `social process'. And
each of which can be used to check the correctness of the entire QED
system.
In any case, it is a highly desireable goal that a checker for the
root logic can be easily written in common programming languages. The
specification should be so unambiguous that many can easily implement
it from its specification in a few pages of code, with total
comprehension by a single person.
It has been argued that the idea of having multiple logics in addition
to the root logic is a mistake that will result in too much
complexity, and that it would be far more sensible to have a single
logic in which proofs were clearly flagged with an indication of the
assumptions used, so that a single logic could be enjoyed by people of
both classical and constructive persuasions. Certainly such a single
logic is desireable, but whether such a single logic can be developed
is a serious question given that some famous constructive theorems
(such as the continuity of all functions on the reals) are classical
falsehoods.
It has been argued that the idea of searching for a single logic or a single
computer system is inferior to the idea of developing translation mechanisms
that would permit proof checking systems to exchange proofs with one another.
If this were feasible, it would certainly permit an alternative, distributed
approach to achieving the major QED objectives. However, the history of
radical incompatibility of many proof checking systems does suggest that such
translation mechanisms may be difficult to produce.
In seeking a root logic, it is clear that there will be many controversies that
will be impossible to resolve to everyone's satisfaction. For example, there
seems no hope of satisfying in a single logic those who insist upon a typed
syntax and those who loathe typed syntax, preferring to do typing internally,
e.g., with sets. There are also simple questions not yet resolved after
centuries of thought, such as the semantics of a function applied outside its
domain, e.g., division by zero.
6. What Is To Be Done?
The idea is to make a language such that everything we
write in it is interpretable as correct mathematics ...
This may include the writing of a vast mathematical
encyclopedia, to which everybody (either a human or a
machine) may contribute what he likes. The idea of a kind
of formalized encyclopedia was already conceived and
partly carried out by Peano around 1900, but that was still
far from what we might call automatically readable.
-- N. G. de Bruijn
Leadership. It seems certain that inviting deliberation by many
interested parties at the planning stage is important not only to get
the QED project off on a correct footing but also to encourage many to
participate in the project. Until we can establish general agreement
within a large, critical mass of scientists (including many
distinguished mathematicians) that the QED project is probably worth
doing, and until a basic `manifesto' agreeable to them can be drafted,
possibly using parts of this document as a starting point, it is not
clear whether there will be any further progress on this project.
Given the extraordinary scope of this project, it is also essential
that research agency leadership be obtained. It is perhaps unlikely
that any one agency would be willing to undertake the funding of the
entirety of such a large project. So an agreement by many agencies to
cooperate will probably be essential. The requirements for
leadership, both by scientists and by research agencies, are so major
that it is perhaps premature to speculate about what other things
should be done, in what order. Nevertheless, we will speculate about
a few issues.
What planning steps should be taken to start the QED project? An
obvious first concern is to enumerate and describe in some detail the
kinds of things that would be found in the QED system, including
logics
axioms
definitions
theorems (including an analysis of the major parts of mathematics)
proofs
proof-checkers
decision procedures
theorem-proving programs
symbolic computation procedures
modeling software
simulation software
tools for experimentation
numerical analysis software
graphical tools for viewing mathematics
interface tools for using the QED system
Crucial to this initial high level organization effort is deciding what parts
of mathematics will be represented, how that mathematics will be organized, and
how it will be presented. It is conceivable that years of consideration of
these points should precede implementation efforts. One can imagine that a
re-organization of mathematics on the order of the scope of the Bourbaki
project is necessary. One can imagine major projects in the development of
formal `higher-level' languages in which mathematics can be formally discussed
and major projects devoted simply to writing the most important theorems,
definitions, and proof sketches in such languages. Because different proofs of
the same theorem can differ substantially in complexity, and because entering
formal proofs into a proof checking system is very expensive, it is highly cost
effective to consider many proofs of a theorem before setting out to verify one
of them. It has been suggested by several people that a useful and relatively
easy early step would be to assemble, in ftp-able form, a comprehensive survey
of the parts of mathematics have been checked by various automated reasoning
systems.
A second planning step would be to establish some `milestones' or some priority
list of objectives. For example, one could attempt to outline which parts of
mathematics should be added to the system in what order. Simultaneously, an
analysis of what sorts of cooperation and resources would be necessary to
achieve the earlier goals should be performed.
A third planning step would be to accumulate the basic mathematical
texts that are to be formalized. It is entirely possible that the QED
project will greatly overlap with an effort to build an electronic
library of mathematical information. It is not part of the idea of a
library that the documents should be in any particular language or
subjected to any sort of rigor check. But it would of great inherent
value, and great value to the QED project, to have the important works
of mathematics available in machine readable form and organized for
ease of access.
A fourth planning step would be to attempt to achieve consensus about
the statement of the most important definitions and theorems in
mathematics. Until there is agreement on the formalization of the
basic concepts and theorems of the important parts of mathematics, it
will be hardly appropriate to begin the difficult task of building
formal proofs of theorems. The formalization of statements is an
extremely difficult and error-prone activity.
Although the scientific obstacles to building QED are formidable, the
social, psychological, political, and economic obstacles seem much
greater. In principle, we can imagine a vast collection of people
successfully collaborating on such an effort. But the problems of
actually getting such a collaboration to occur are possibly
insurmountable. `Why,' an individual researcher could well ask,
`should I risk my future by working on what will be but a small part
of a vast undertaking? What sort of recognition will I receive for
contributing to yet one more computing system?' These are good
questions, and it is not clear what the answer is. To a major extent,
status in mathematics and computing is a function of publications in
major journals -- status for research funding, status for tenure
decisions, status for promotion. It is far from clear how
contributing pieces to the QED system could provide a substitute for
such signs of status. Perhaps here research agencies or even
university faculties and administrators could be of assistance in
establishing a new societal framework in which such cooperation was
encouraged.
Even given the cooperation of all the necessary people and assuming
good luck in overcoming scientific hurdles, there are many issues of a
very difficult but somewhat mundane character involving: version
control, distribution, and support. A system with hundreds of
contributors will create management difficulties perhaps not even
imaginable to the small groups of researchers who have worked in the
past on parts of the QED idea.
It has been suggested about the low-level QED data files that they
should be humanly readable and permit comments, and that the character
set should be email-able.
Non-Copyright: This document is in the public domain and so unlimited
alteration, reproduction, and distribution by anyone are permitted.
Authorship: This preliminary discussion of project QED (very tentative
name) is an amalgam of many ideas that many people have had and for
which perhaps no one alive today deserves much credit. We are
deliberately avoiding any authorship or institutional affiliation at
this early stage in the project (and may decide to do so forever) in
the hope that many will want to join in the project as principals,
even as originators (to the extent that anyone alive today could be
thought to be an originator of this project). Some of those involved
in the project would much rather that QED be completed than that they,
as individuals, be lucky enough to partake significantly in the
project, much less get any public credit for its completion. It may
seem paranoid to avoid personalities, but we are inspired by the
extraordinary cooperation achieved in the Bourbaki series in an
atmosphere of anonymity.
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