Let us, then, try once more, so as not to involve the acquaintance with a a concept of X word in the having (3) We hav when we are able to distinguish X's from Y's and Zs and indeed from that is not an x. We might this whether we had a word for x or not, though of course it convenient if we did have a word and normally we would have. Thus if a child can distinguish cats from dogs and pigs and all other things, he has a concept of what is a cat, even though he cannot state a definition and even though he has never heard the word "cat and connected the word with the thing by way of ostensive definition We have now specified what a concept is in such a way as to make i possible to have a concept without knowing any words. A dog that can distinguish cats from birds can be said to have these concepts, although it knows no words. Even this definition might be objected to, however, on the ound that being able to distinguish X's from Y's is, once again consequence of having the concept of X, but not what having the concept consists in. One is tempted to say that if you have a concept of X, you can, as a result, distinguish X's from other things, but you have to have the concept first. But what then would having the concept be? Moreover, we can devise machines that can effectively differentiate some things from others; do we wish to say that these machines have concepts? In reply to such objections, we might say (4) that to have a concept of X is simply to have some criterion-in-mind. It would consist in some kind of "mental content" quite independent of words and quite independent of distinguishing X's from Y's and Z's. But it is not easy to state what such criterion-in-mind would be like, or how one would know, through introspection alone, whether one possessed such a criterion. Surely the way one would know whether one had a criterion for X would be whether it would enable one to distinguish X's from Y's and Z's. A criterion for identifying X's would (it would seem) automatically be a criterion for distinguishing X's from non-X's. And so we would be back with our third criterion after all