So let us try again: (2) We have a concept of X when we can apply the word "X" correctly; we have a concept of redness and orangeness when we can correctly apply the words "red" and "orange" in all cases. This criterion does not require us to give a definition but only to use the word with uniform correctness It is also much more in line with our actual use of the term "concept": we do say, for example, "He must have some concept of what a cat is, for he always uses the word "cat" in the right situations he never applies the word "cats' to dogs or anything else.