Really? The words "dogs" refers to something like a set whereas "dog" (usually) refers to an individual. Thus they have very different meanings, even if those

Really? The words "dogs" refers to something like a set whereas "dog" (usually) refers to an individual. Thus they have very different meanings, even if those meanings have an obvious and simple relationship. On the other hand "cat" and "dog" refer to individuals that are similar in many ways (and interchangeable for some common purposes). That is, these two words have somewhat similar meanings.