# [FOM] Theory of Actual Proofs

Harvey Friedman friedman at math.ohio-state.edu
Tue Jun 17 14:00:57 EDT 2003

```Reply to Guglielmi 2:22PM 6/16/03.

>I don't believe that linear logic has a direct relevance for the
>foundations of mathematics, and, at least as it stands now, it is
>different than intuitionistic logic, or relevance logic: these
>logics come with a philosophy, they aim at being foundational.
>Linear logic is just an algebraic theory.
>
>On the other hand, I believe that linear logic might have a
>beneficial indirect effect on the foundations of mathematics, and
>could be further developed to become a foundation for computer
>science.

I think all subscribers appreciate this frank assessment. I now feel
less guilty/stupid for having ignored linear logic.

>
>I'd say that proof theory is still seriously underdeveloped, as
>witnessed by the fact that we are still not able to answer the
>question: `When are two different mathematical proofs essentially
>the same?' (it's not even clear how to define being `essentially the
>same'); and supposing we know that: `Given two proofs, can we decide
>that they are the same?'.
>
>One of the reasons for our inability is that proof theory is
>currently suffocated by bureaucracy, meaning that proofs are
>cumbersome objects, which leave us very little freedom of
>manipulation and analysis.

Let me mention a possible approach to this problem of equivalence
between proofs - and also the problem of bureaucracy.

First of all, one should aim for informative equivalence relations
between proofs, without having to assert that one has really found
THE notion of identity between proofs.

I have been working on sorting out notions of trivial proof. These
are proofs that have no ideas, and are forced upon us by routine
unraveling of the theorem being proved. I will be more precise later
this later, as there are some crucial ambiguities that need to be
clarified here. In any case, all trivial proofs will be regarded as
the same proof - the trivial proof.

So what significant equivalence relation do we want to place on proofs?

My proposal of the condition to study is this. Every assertion made
in the first proof has a trivial proof from the set of assertions
made in the second proof, and every assertion made in the second
proof has a trivial proof from the set of assertions made in the
first proof.

There is a problem conducting serious research on such ideas, and
even the more elemental problem of uncovering appropriate notions of
"trivial proof".

There is only one real source of representative proofs of varying
depth that is appropriate for such research. It is from actual
elementary mathematics. State of the art mathematics is much too
unwieldy for this kind of study (at this time), and there is already
much more than enough in actual elementary mathematics.

The trouble is that even quite elementary mathematics, when fully
formalized, is unwieldy and unreadable unless one is casting it in a
particularly flexible formal language.

One can try to provide the most convenient possible formal language,
equipped with rigorous semantics, for the practicing mathematician,
where validity is r.e., of course. This is a very worthy project,
which is particularly vital for creating, say, a database of
mathematical information. The problem is, of course, to do this so
well that a mathematician can profitably search such a database. I
believe that this can be, and will be, done so well as to be a useful
tool for mathematicians. But that is another project.

Coming back to the matter at hand, it will be sufficient to use a
reasonably sophisticated form of first order ZFC for this study.
There is a happy medium that is enough so that one can study serious
examples of actual proofs.

After the construction of this form of ZFC, which flexibly supports
undefined terms, and abbreviations, the first issue that comes up is:
what is a mathematical theorem?

A mathematical theorem is not simply a sentence (or formula) in the
language of set theory that is provable in ZFC. Serious mathematical
theorems can rarely be stated in such primitive notation.

Instead, I take it to be given by a finite sequence of formulas,
where all entries in the sequence except the last entry, is the
introduction of a relation, constant, or function by explicit
definition. (Allowing undefined terms alleviates any responsibility
for proving existence or uniqueness when making these definitions).
The final entry in the sequence is the actual theorem, and is given
using only the symbols previously defined in the sequence, together
with the primitives of ZFC.

The theorem is considered valid if and only if every model of ZFC
plus all of the entries except the last, is a model of the last entry.

If this is to be used as a model of mathematical practice, then we
consider the  stuff above embedded in a wider structure.

Thus we come to the notion of a Mathematical Development over ZFC. A
MD over ZFC has absolutely no proofs. Instead, it is a sequence of
formulas, every one of which is

i) an explicit definition of a new symbol using only symbols that
have appeared previously or are the primitives of ZFC. These are
labeled DEFINITION. Or,

ii) a formula using only symbols that have appeared previously, and
is labeled THEOREM.

The MD over ZFC is considered valid if and only if every statement
labeled THEOREM holds in every model of ZFC in which all previous
entries labeled DEFINITION hold. Inductively, this is the same as
requiring that every statement labeled THEOREM holds in every model
of ZFC in which all previous entries hold.

Now how are we going to talk about proofs, and in particular trivial
proofs? Or the level of triviality of a proof?

I use the Beth tableaux idea as a uniform gold standard. I am hereby
going to collectively call of the relevant variants of Beth tableaux
that are important in this approach, model trees. I say model trees
because they are exactly the trees you would most naturally use to
try to construct countermodels. Remember that a formula is valid if
and only if various associated model trees have no infinite path if
and only if various associated model trees are finite.

The proper use of model trees allows us to "write" proofs with almost
no symbols, relieving us of nearly all of the "bureaucracy" that you
refer to. The only thing that needs to be written down is what
corresponds to the nontrivial "idea". And to verify that the mere
writing down of the "ideas" in proper form, results in a correct
proof, is best done by a piece of software that generates the
required model trees. Given the setup, one knows in advance that the
construction of the appropriate model trees is efficient - otherwise
an "idea" is missing.

When this is set up properly, one can then redo elementary
mathematics in this way, where only ideas appear in the human proof.
The rest is taken care of by the software.

Of course, the main "ideas" needed are often the citation of relevant
previously established facts, including the specification of
instances of schemes.

I never liked schemes here, but I think that they are just too useful
a tool to remain unused for such research.

Enough for now. I would be interested to see if people are interested...

PS: There is one arena where these ideas may need some significant
sharpening. That is, in the arena of algebraic manipulations. It may
be somewhat unsatisfactory to have to quote just what algebraic facts
(equalities and inequalities) one is using. Rather, one may wish to
import packages into the picture. Packages are essential for normal
automated theorem proving projects of various kinds, but that's not
what we are talking about. We are talking about a theory of actual
proofs without "bureaucracy", something quite different.

Harvey Friedman
```