Talk:Categorical logic

WikiProject class rating
This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 03:50, 10 November 2007 (UTC)

Ambiguous title?
Some introductory logic texts use the term "Categorical Logic" to refer to plain, ol' Aristotelian logic, which has nothing to do with category theory (aside from the fact that category theory can be used to understand everything). I've taught from two texts which use this terminology. Do you suppose that the article should acknowledge this ambiguity and give readers a link to the appropriate article? (Which article?) Phiwum (talk) 13:36, 28 May 2009 (UTC)

The resulting logic is formally intuitionistic
I removed this sentence from the first paragraph of Historical perspective because no citation is provided. Although this is a widely held opinion, it is false. Colin McLarty, a real authority on the subject, says on the first page of the preface to his book Elementary Categories, Elementary Toposes:
 * In Part III the first order topos axioms are used to define the higher order internal language. The logic is called 'topos logic' here, although 'intuitionistic logic' is often referred to in the literature. I believe that is usually, and rightly, taken to mean Brouwer's epistemology of mathematics, which is unrelated to the origin or content [emphasis mine] of topos theory. Topos logic strongly resembles formal intuitionistic logic and the two have interacted, as in .... But topos logic coincides with no intuitionistic logic studied before toposes, and this is due to real philosophical differences..."--Foobarnix (talk) 19:36, 5 July 2015 (UTC)

Paragraph removed from Historical perspective section

 * I removed the entire paragraph below from the Historical perspective section:
 * To go back historically, the major irony here is that in large-scale terms, intuitionistic logic had reappeared in mathematics, in a central place in the Bourbaki–Grothendieck program, a generation after the messy Brouwer–Hilbert controversy had ended, with Hilbert the apparent winner. Bourbaki, or more accurately Jean Dieudonné, having laid claim to the legacy of Hilbert and the Göttingen school including Emmy Noether, had revived intuitionistic logic's credibility (although Dieudonné himself found Intuitionistic Logic ludicrous), as the logic of an arbitrary topos, where classical logic was that of 'the' topos of sets. This was one consequence, certainly unanticipated, of Grothendieck's relative point of view; and not lost on Pierre Cartier, one of the broadest of the core group of French mathematicians around Bourbaki and IHES. Cartier was to give a Séminaire Bourbaki exposition of intuitionistic logic.


 * Reasons for removal
 * As we've seen from McLarty, intuitionist logic is not topos logic [I could give other sources]. The "had" is the wrong tense anyway. There is no such thing as "the Bourbaki-Grothendieck program" and if there were, it would have almost nothing to do with categorical logic. Grothendieck was unaware of the relations of his topos theory to logic; the relation was entirely the discovery of Lawvere. Briefly, G invented what he called a "topos" for his work in algebraic geometry; today one speaks of a "Grothendieck topos". The term "topos" usually refers to what is sometimes called an "elementary topos" (that's what Lawvere called it), a generalization of G. toposes due to Lawvere, with the aid of Tierney. A "Grothendieck topos" is not the same as an ("elementary" or "Lawvere-Tierney") "topos", and they are never confused.


 * As for "Jean Dieudonné, having laid claim to the legacy of Hilbert", this is totally false. Dieudonné worked in many fields, but never in logic. Dieudonné reviving intuitionist logic's credibility? This is totally false. Brouwer's views on logic were given a respectable interpretation by his student Arend Heyting. Andrey Kolmogorov came up with approximately the same logic from a quite different viewpoint. Today one speaks of the BHK-interpretation (Brouwer-Heyting-Kolmogorov). Again, neither Dieudonné, nor any other Bourbakiste, had anything whatever to do with this.--Foobarnix (talk) 20:15, 5 July 2015 (UTC)

Copyright violations
I removed the section Historical perspective. All of it is a copyright violation. The exact text can be found on pages 2 to 5 of "A Functorial Model Theory: Newer Applications to Algebraic Topology, Descriptive Sets, and Computing Categories Topos" by Cyrus F. Nourani,   ISBN 1926895924

See the original text here.

Frege, semigroups, and the categorical view
This article is OK as far as it goes. But I think it needs to be cleaned up a bit. I am not personally competent to do this (I tried). But I invite someone who is more knowledgeable than me to use some version of the remarks below to improve the article.

The article should say that classically (i.e. before categorical logic), logic dealt with interpretations in the category of non-empty sets and functions., so that e.g. it could not handle semi-groups since there is an empty semi, group. This limitation was due to an improper understanding of how to handle free variables: the categorical view shows that one must consider a term or formula or sequent not by itself but together with a list of variables that contains all the free variables of a term, formula, or sequent and possible some other variables; this list is called the context. Perhaps most importantly, however, is that pre-categorical logic inherited from Frege the requirement that variables vary over the entire universe, and consequently that equality was "universal", in the sense that t "x = y" was considered a meaningful statement, no matter what x and y are. In contrast, categorical logic is generally "typed", so that each variable ranges over one type only (leaving aside the possibility of subtyping), and "x = y" is an allowable statement only if x and y are of the same type. [It may be that this last part is too vague because of the omission of subtyping].--Toploftical (talk) 14:44, 16 December 2015 (UTC)

Assessment comment
Substituted at 19:51, 1 May 2016 (UTC)

Removed Philosophy Rating
This was rated as of High Importance in Philosophy. That would be true if this article were Aristotle's Categorical Logic (linked as Term Logic), which has been a central part of Western Philosophy's tradition for 2400 years. But it's not, and methamatical Categorial Logic is not of any great importance to Philosophy, except perhaps indirectly through its role in Mathematics. If someone feels that the Philosophy Rating box should be reverted, it should be Low. But better it's not there. Unless all of Mathematics is part of Philosophy.