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This post is a continuation of the discussion of “the elementary theory of the category of sets” [ETCS] which we had begun last time, here and in the comments which followed. My thanks go to all who commented, for some useful feedback and thought-provoking questions.

Today I’ll describe some of the set-theoretic operations and “internal logic” of ETCS. I have a feeling that some people are going to love this, and some are going to hate it. My main worry is that it will leave some readers bewildered or exasperated, thinking that category theory has an amazing ability to make easy things difficult.

**An****aside**: has anyone out there seen the book Mathematics Made Difficult? It’s probably out of print by now, but I recommend checking it out if you ever run into it — it’s a kind of extended in-joke which pokes fun at category theory and abstract methods generally. Some category theorists I know take a dim view of this book; I for my part found certain passages hilarious, in some cases making me laugh out loud for five minutes straight. There*are*category-theory-based books and articles out there which cry out for parody!

In an attempt to nip my concerns in the bud, let me remind my readers that there are major differences between the way that standard set theories like ZFC treat membership and the way ETCS treats membership, and that differences at such a fundamental level are bound to propagate throughout the theoretical development, and impart a somewhat different character or feel between the theories. The differences may be summarized as follows:

- Membership in ZFC is a
*global*relation between objects of the*same*type (sets). - Membership in ETCS is a
*local*relation between objects of*different*types (“generalized” elements or functions, and sets).

Part of what we meant by “local” is that an element *per se* is always considered relative to a particular set to which it belongs; strictly speaking, as per the discussion last time, the same element is never considered as belonging to two different sets. That is, in ETCS, an (ordinary) element of a set is defined to be a morphism ; since the codomain is fixed, the same morphism cannot be an element of a different set . This implies in particular that in ETCS, there is no meaningful global intersection operation on sets, which in ZFC is defined by:

Instead, in ETCS, what we have is a *local* intersection operation on *subsets* of a set. But even the word “subset” requires care, because of how we are now treating membership. So let’s back up, and lay out some simple but fundamental definitions of terms as we are now using them.

Given two monomorphisms , we write ( if the monos are understood, or if we wish to emphasize this is local to ) if there is a morphism such that . Since is monic, there can be at most one such morphism ; since is monic, such must be monic as well. We say *define the same subset* if this is an isomorphism. So: subsets of are defined to be isomorphism classes of monomorphisms into . As a simple exercise, one may show that monos into define the same subset if and only if and . The (reflexive, transitive) relation on monomorphisms thus induces a reflexive, transitive, antisymmetric relation, i.e., a partial order on subsets of .

Taking some notational liberties, we write to indicate a subset of (as isomorphism class of monos). If is a generalized element, let us say *is in* a subset if it factors (evidently uniquely) through any representative mono , i.e., if there exists such that . Now the *intersection* of two subsets and is defined to be the subset defined by the pullback of any two representative monos . Following the “Yoneda principle”, it may equivalently be defined up to isomorphism by specifying its generalized elements:

Thus, intersection works essentially the same way as in ZFC, only it’s *local* to subsets of a given set.

While we’re at it, let’s reformulate the power set axiom in this language: it says simply that for each set there is a set and a subset , such that for any relation , there is a unique “classifying map” whereby, under , we have

The equality is an equality between subsets, and the inverse image on the right is defined by a pullback. In categorical set theory notation,

Hence, there are natural bijections

between subsets and classifying maps. The subset corresponding to is denoted or , and is called the *extension* of .

The set plays a particularly important role; it is called the “subset classifier” because subsets are in natural bijection with functions . [Cf. classifying spaces in the theory of fiber bundles.]

In ordinary set theory, the role of is played by the 2-element set . Here subsets are classified by their characteristic functions , defined by iff . We thus have ; the elementhood relation boils down to . Something similar happens in ETCS set theory:

**Lemma 1: **The domain of elementhood is terminal.

**Proof: **A map , that is, a map which is in , corresponds exactly to a subset which contains all of (i.e., the subobject ). Since the only such subset is , there is exactly one map .

Hence elementhood is given by an element . The power set axiom says that a subset is retrieved from its classifying map as the pullback .

Part of the power of, well, power sets is in a certain dialectic between external operations on subsets and internal operations on ; one can do some rather amazing things with this. The intuitive (and pre-axiomatic) point is that if has finite products, equalizers, and power objects, then is a representing object for the functor

which maps an object to the collection of subobjects of , and which maps a morphism (“function”) to the “inverse image” function , that sends a subset to the subset given by the pullback of the arrows , . By the Yoneda lemma, this representability means that *external *natural operations on the correspond to *internal* operations on the object . As we will see, one can play off the external and internal points of view against each other to build up a considerable amount of logical structure, enough for just about any mathematical purpose.

**Remark**: A category satisfying just the first three axioms of ETCS, namely existence of finite products, equalizers, and power objects, is called an (elementary)**topos**. Most or perhaps all of this post will use just those axioms, so we are really doing some elementary topos theory. As I was just saying, we can build up a tremendous amount of logic internally within a topos, but there’s a catch: this logic will be in general intuitionistic. One gets classical logic (including law of the excluded middle) if one assumes strong extensionality [where we get the definition of a well-pointed topos]. Topos theory has a somewhat fearsome reputation, unfortunately; I’m hoping these notes will help alleviate some of the sting.

To continue this train of thought: by the Yoneda lemma, the representing isomorphism

is determined by a universal element , i.e., a subset of , namely the mono . In that sense, plays the role of a *universal subset*. The Yoneda lemma implies that external natural operations on general posets are completely determined by how they work on the universal subset.

**Internal Meets**

To illustrate these ideas, let us consider intersection. Externally, the intersection operation is a natural transformation

This corresponds to a natural transformation

which (by Yoneda) is given by a function . Working through the details, this function is obtained by putting and chasing

through the composite

Let’s analyze this bit by bit. The subset is given by

and the subset is given by

Hence is given by the pullback of the functions and , which is just

The map is thus defined to be the classifying map of .

To go from the internal meet back to the external intersection operation, let be two subsets, with classifying maps . By the definition of , we have that for all generalized elements

if and only if

(where the equality signs are interpreted with the help of equalizers). This holds true iff is in the subset and is in the subset , i.e., if and only if is in the subset . Thus is indeed the classifying map of . In other words, .

A by-product of the interplay between the internal and external is that the internal intersection operator

is the meet operator of an internal meet-semilattice structure on : it is commutative, associative, and idempotent (because that is true of external intersection). The identity element for is the element . In particular, carries an internal poset structure: given generalized elements , we may define

if and only if

and this defines a reflexive, symmetric, antisymmetric relation :

equivalently described as the equalizer

of the maps and . We have that if and only if .

**Internal Implication**

Here we begin to see some of the amazing power of the interplay between internal and external logical operations. We will prove that carries an internal Heyting algebra structure (ignoring joins for the time being).

Let’s recall the notion of Heyting algebra in ordinary naive set-theoretic terms: it’s a lattice that has a material implication operator such that, for all ,

if and only if

Now: by the universal property of , a putative implication operation is uniquely determined as the classifying map of its inverse image , whose collection of generalized elements is

Given , the equality here is equivalent to

(because is maximal), which in turn is equivalent to

This means we should define to be the classifying map of the subset

**Theorem 1: ** admits internal implication.

**Proof: **We must check that for any three generalized elements , we have

if and only if

Passing to the external picture, let be the corresponding subsets of . Now: according to how we defined a generalized element is in if and only if . This applies in particular to any monomorphism that represents the subset .

**Lemma 2:** The composite

is the classifying map of the subset .

**Proof:** As subsets of , where the last equation holds because both sides are the subsets defined as the pullback of two representative monos , .

Continuing the proof of theorem 1, we see by lemma 2 that the condition corresponds externally to the condition

and this condition is equivalent to . Passing back to the internal picture, this is equivalent to , and the proof of theorem 1 is complete.

**Cartesian Closed Structure**

Next we address a comment made by “James”, that a category satisfying the ETCS axioms is cartesian closed. As everything else in this article, this uses only the fact that such a category is a topos: has finite products, equalizers, and “power sets”.

**Proposition 1:** If are “sets”, then represents an exponential

**Proof:** By the power set axiom, there is a bijection between maps into the power set and relations:

which is natural in . By the same token, there is a natural bijection

Putting these together, we have a natural isomorphism

and this representability means precisely that plays the role of an exponential .

**Corollary 1:** .

The universal element of this representation is an evaluation map , which is just the classifying map of the subset .

Thus, represents the set of all functions (that is, relations from to ). This is all we need to continue the discussion of internal logic in this post, but let’s also sketch how we get full cartesian closure. [Warning: for those who are not comfortable with categorical reasoning, this sketch could be rough going in places.]

As per our discussion, we want to internalize the set of such relations which are graphs of functions, i.e., maps where each is a singleton, in other words which factor as

where is the singleton mapping:

We see from this set-theoretic description that classifies the equality relation

which we can think of as either the equalizer of the pair of maps or, what is the same, the diagonal map .

Thus, we put , and it is not too hard to show that the singleton mapping is a monomorphism. As usual, we get this monomorphism as the pullback of along *its* classifying map .

Now: a right adjoint such as preserves all limits, and in particular pullbacks, so we ought then to have a pullback

B^A ---------------> 1^A | | sigma^A | | t^A V V P(B)^A -------------> P(1)^A (chi_sigma)^A

Of course, we don’t even have yet, but this should give us an idea: *define* , and in particular its domain , by taking the pullback of the right-hand map along the bottom map. In case there is doubt, the map on the bottom is defined Yoneda-wise, applying the isomorphism

to the element in the hom-set (on the left) given by the composite

The map on the right of the pullback is defined similarly. That this recipe really gives a construction of will be left as an exercise for the reader.

**Universal Quantification**

As further evidence of the power of the internal-external dialectic, we show how to internalize universal quantification.

As we are dealing here now with *predicate* logic, let’s begin by defining some terms as to be used in ETCS and topos theory:

- An
*ordinary predicate*of type is a function . Alternatively, it is an ordinary element . It corresponds (naturally and bijectively) to a subset . - A
*generalized predicate*of type is a function . It may be identified with (corresponds naturally and bijectively to) a function , or to a subset .

We are trying to define an operator which will take a predicate of the form [conventionally written ] to a predicate [conventionally written ]. Externally, this corresponds to a natural operation which takes subsets of to subsets of . *Internally*, it corresponds to an operation of the form

This function is determined by the subset , defined elementwise by

Now, in ordinary logic, is true if and only if is true for all , or, in slightly different words, if is constantly true over all of :

The expression on the right (global truth over ) corresponds to a function , indeed a monomorphism since any function with domain is monic. Thus we are led to define the desired quantification operator as the classifying map of .

Let’s check how this works externally. Let be a generalized predicate of type . Then according to how has just been defined, classifies the subset

There is an interesting adjoint relationship between universal quantification and substitution (aka “pulling back”). By “substitution”, we mean that given any predicate on , we can always pull back to a predicate on (substituting in a dummy variable of type , forming e.g. ) by composing with the projection . In terms of subsets, substitution along is the natural external operation

Then, for any predicate , we have the adjoint relationship

if and only if

so that substitution along is left adjoint to universal quantification along . This is easy to check; I’ll leave that to the reader.

**Internal Intersection Operators**

Now we put all of the above together, to define an internal intersection operator

which intuitively takes an element (a family of subsets of ) to its intersection , as a subset .

Let’s first write out a logical formula which expresses intersection:

We have all the ingredients to deal with the logical formula on the right: we have an implication operator as part of the internal Heyting algebra structure on , and we have the quantification operator . The atomic expressions and refer to internal elementhood: means is in , and means is in .

There is a slight catch, in that the predicates “” and “” (as generalized predicates over , where lives) are taken over different domains. The first is of the form , and the second is of the form . No matter: we just substitute in some dummy variables. That is, we just pull these maps back to a common domain , forming the composites

and

Putting all this together, we form the composite

This composite directly expresses the definition of the internal predicate given above. By cartesian closure, this map induces the desired internal intersection operator, .

This construction provides an important bridge to getting the rest of the internal logic of ETCS. Since we can can construct the intersection of arbitrary definable families of subsets, the power sets are internal inf-lattices. But inf-lattices are sup-lattices as well; on this basis we will be able to construct the colimits (e.g., finite sums, coequalizers) that we need. Similarly, the intersection operators easily allow us to construct image factorizations: any function can be factored (in an essentially unique way) as an epi or surjection to the image, followed by a mono or injection . The trick is to define the image as the smallest subset of through which factors, by taking the intersection of all such subsets. Image factorization leads in turn to the construction of existential quantification.

As remarked above, the internal logic of a topos is generally intuitionistic (the law of excluded middle is not satisfied). But, if we add in the axiom of strong extensionality of ETCS, then we’re back to ordinary classical logic, where the law of excluded middle is satisfied, and where we just have the two truth values “true” and “false”. This means we will be able to reason in ETCS set theory just as we do in ordinary mathematics, taking just a bit of care with how we treat membership. The foregoing discussion gives indication that logical operations in categorical set theory work in ways familiar from naive set theory, and that basic set-theoretic constructions like intersection are well-grounded in ETCS.

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