What vs that whichFor reasons known only to the webgoblins, I can't get Haloscan to accept a comment. But I wanted to answer Zero's comment, so here's the reply. Zero first in italics:
Not the conjunction but [what] we are talking about.
[What] doesn't kill us merely prolongs the inevitable.
why wouldn't using "what" be a suitable substitute? euphony?
is it simply a matter of style?
it seems to mean the same thing.
actually, using "what" seems more efficient than using "that which".
Yes. "What" can sometimes be used for "that which" but it is uncomfortable for "the thing that". It can't point out a particular thing but refers to an abstract something. "I don't know what you're talking about" refers to an abstract thing talked about. (But: "Have you read the Information?" "Who's it by?" "Amis." "I've read some of his books, but not that which you're talking about." "What" is impossible in that example. "The one that" is fine. As a guiding rule, use "that which" when it could be substituted by "the one that" and you won't go wrong.)
In the first example we're considering, it's a particular pronoun in question. I would prefer "that which" because it points to the pronoun and not the matter of our discussion, which "what" would imply. In the second, "what" is fine. I'd say "What doesn't kill you makes you stronger" for preference, if I was talking about an abstract thing (because "what" in this instance refers to the entire universe of things that might kill you/make you stronger). But I'd use "that which" if the conversation went like this:
"Which drugs do you recommend?"
"Well, that which doesn't kill you makes you stronger".
This is because I'm referring to "the drug" and not "the entire abstract universe".