Mathematics is generally regarded as the exact subject par excellence. But the language commonly used by mathematicians (in papers, monographs, and even textbooks) can be remarkably vague; perhaps not when compared with everyday speech but certainly when compared with the language used by practitioners of other intellectual disciplines such as the natural sciences or philosophy. [trybulec-z] point out that in a way this is natural: since the underlying semantics of mathematics is generally clearer than that of other disciplines, the mind is naturally trammelled into a precise mode of thinking, and terminological exactness is less important. But a knowledgeable reader is sometimes needed to separate rhetorical flourish from real content, and to appreciate the context in which results are asserted. Some everyday phrases are imbued with a particular significance; observe for example the crucial distinction between a 'mapping into X' and a 'mapping onto X', or the precise (albeit context-dependent) meaning of woolly-sounding expressions like 'for almost all x'. And on the other hand many issues that the author feels are obvious or unimportant may be glossed over. [bourbaki-sets] stresses the importance of ' abuses of language, without which any mathematical text runs the risk of pedantry, not to say unreadability', but many of the conventions go beyond mere abuse of language. Consider the following examples taken from the early pages of [matsumura]:
[trybulec-z] remark that the language of mathematical texts isn't incontrovertibly a natural language at all; but it invariably contains a substantial admixture of natural language, and that has all the usual potential for ambiguity and imprecision. This might not be a problem if mathematics were a small, unified subject easily graspable by a single practitioner. But on the contrary mathematics is heading increasingly in the direction of specialization, and it's not realistic to expect mathematical physicists, say, to appreciate deeply all the theoretical results in topology, differential geometry, numerical analysis and what not that they use.
There is also the question of the correctness of mathematical reasoning. Mathematical proofs are subjected to peer review before publication, but there are plenty of well-documented cases where published results turned out to be faulty. A notable example is the purported proof of the 4-colour theorem by [kempe]; the error in this proof was eventually pointed out in print by [heaywood], and it is only with the work of [appel-4] that the theorem has finally come to be accepted.1 The errors need not be deep mathematical ones, as shown by the following from [littlewood-miscellany]:
Professor Offord and I recently committed ourselves to an odd mistake (Annals of Mathematics (2) 49, 923, 1.5). In formulating a proof a plus sign got omitted, becoming in effect a multiplication sign. The resulting false formula got accepted as a basis for the ensuing fallacious argument. (In defence, the final result was known to be true.)
A book by [lecat-errors] gave 130 pages of errors made by major mathematicians up to 1900. With the abundance of theorems being published today, often emanating from writers who are not trained mathematicians, one fears that a project like Lecat's would be practically impossible, or at least would demand a journal to itself!
We have separated the concerns of imprecision and incorrectness, but they are linked. If one mathematician uses another's result without fully appreciating the definitions underlying it or the conditions attached to it, this can lead to errors. Moreover, as famously shown by [lakatos-pr], incorrectness and imprecision can shade imperceptibly close to one another. Lakatos discusses how Euler's theorem about polyhedra has changed over time. It was shown to be incorrect, but rather than being overthrown, it merely evolved, incorporating additional explicit conditions which formerly were either left unsaid or not appreciated at all. In the final incarnation its validity stands almost as a definition of a certain kind of polyhedron. This is not such an uncommon fate for important theorems. For example, Stokes's theorem has been generalized so much, and been associated with the development of so much technical machinery, that the theorem itself is almost a triviality. Even if one does not subscribe in full to Lakatos's thesis that mathematics has a strong empirical component placing it almost on a par with physical sciences, such examples are worth reflecting on.
The formalization of mathematics addresses both these questions, precision and correctness. By formalization we mean expressing mathematics, both statements and proofs, in a (usually small and simple) formal language with strict rules of grammar and unambiguous semantics. The latter point bears repeating: just writing something in a symbolic language is not sufficient for formality if the semantics of that language is ill-defined.2 We can split the project of formalization into two parts:
These are usually considered together, but perhaps merely stating theorems formally without proving them has itself a valuable role. In the construction of computer systems, formal specification has proved to be valuable in bringing out unstated assumptions and clarifying thoughts --- it doesn't seem too fanciful to suggest that there could be similar benefits for mathematics. In practice, though, we are also interested in checking the correctness of proofs, and formalization of the statements and then the proofs is a prerequisite for doing this systematically.