"Consider the bear that/which scratches his head." Which is correct?

If I wish to say something along the lines of

Consider the bear that scratches his head.

It seems to me that I could instead say

Consider the bear which scratches his head.

I am unsure which of these is correct, if it even matters.

Does anyone know a rule which makes this clear?

Solution 1:

That is restrictive, it limits / restricts / specifies the identity of the subject. Using your example, the bear that scratches his head refers to one specific bear -- "the bear that scratches his head".

Which is non-restrictive, meaning it refers to something incidental about the subject. "Consider the bear, which scratches its head" refers to the bear (could be a single bear, could be the species), which happens to scratch its head.

Hope that helps!

EDIT: ShreevatsaR has pointed out that this is a convention, not a grammar rule. In the end it doesn't "matter", use the convention if it appeals to you. Here is MW's take (thanks, nohat).

Solution 2:

What the other answers have said about "which" having to be used with commas (or in non-restrictive clauses) is wrong. "Which" has long been used by respectable writers in restrictive clauses as well. And when it's used in a restrictive clause, it's wrong to use a comma before it. There's a separate question about this: When to use “that” and when to use “which”?

Consider the bear which scratches his head.

This sentence does sound wrong, but for another reason. "Which," whether used restrictively or nonrestrictively, has a strong tendency to be used with inanimates. For this reason, "which... his" sounds bad, because "which" implies you're thinking of the bear as an object, or at least as a not-very-animate thing, while "his" implies you're thinking of the bear as a person, or at least as a somewhat animate being. "Consider the bear who scratches his head" would be better, or, as the other answers mentioned, you could use "that" and say "Consider the bear that scratches his head." (In general, "that" is not as common as "who" when referring to people, but either is grammatical.)

ShreevatsaR in a comment mentioned a third option, "the bear which scratches its head." This doesn't sound as clashingly bad to me as "which scratches his head," but it also doesn't sound as good to me as any of the alternatives I listed in the previous paragraph.