Nietzche was, of course, wrong. Plenty of things that do not kill us make us weaker. Like arcane grammar rules.
You probably know there’s a special rule about the relative pronouns which and that, but you can’t quite remember it. Let’s figure this out.
When do I use which?
Use which to introduce extra information about what comes before it.
I love Star Wars: A New Hope, which came out when I was 11.
The clause “which came out when I was 11” provides extra information about Star Wars: A New Hope. You know this information is extra because you could drop it and the sentence would still make sense.
I love Star Wars: A New Hope.
When a clause adds extra information, set it off with commas.
The Force, which is an energy field, is strong with young Skywalker.
When do I use that?
Use that to introduce a clause that defines what came before it.
Star Wars was the movie that summed up my childhood.
The clause “that summed up my childhood” defines the word “movie.” It is necessary, not extra. If you dropped it, you’d have a say-nothing sentence.
Star Wars was the movie.
Here’s an even clearer example:
The movie that came out in 1977 is just as good as the movie that came out in 1980.
If you dropped “that came out in 1977” and “that came out in 1980,” the sentence would be nonsense:
The movie is just as good as the movie.
Since these clauses are necessary, they should not be set off with commas.
What’s the rule about leaving out that?
If that is the subject of the clause, you must keep that.
My 11-year-old eyes couldn’t believe the ships that flew across the screen.
My 11-year-old eyes couldn’t believe the ships flew across the screen.
If that starts a clause with a different subject, you can leave out that.
I thought that they were real.
I thought they were real.
Either of these is correct. The first is more formal and more common in written English, and the second is less formal and more common in spoken English.
What about that which?
You really are a brave grammarian, asking a question like that.
Nietzche famously said, “That which doesn’t kill us makes us stronger.”
According to the rules we’ve learned so far, though, the sentence should read this way:
Nietzche famously said, “That that doesn’t kill us makes us stronger.”
What an absurd thing to say! And not just grammatically.
What’s going on here? The subject of Nietzche’s sentence is the first “That,” but the clause that follows it is necessary. If you dropped it, you’d have a hobbled idea.
Nietzche famously said, “That makes us stronger.”
So, the sentence begins with “That,” and the clause also should begin with “that,” but writing it that way sounds confusing and clumsy. So philosophers and poets use “that which.”
That which is called a rose
By any other name would smell as sweet.
Normal human beings tend to use “whatever.”
But I famously said, “Whatever doesn’t kill us can maim us for life.”
Why don't you confuse me further?
Sure. These two words can also function as adjectives.
I’m not sure I needed to learn that rule.
You’re not sure you needed to learn which rule?
They can also just be stand-alone pronouns.
That is a confusing rule.
Which is a confusing rule?
So, those other rules about which and that apply only if they introduce a relative clause.
Do you feel stronger? I thought not.
Take that, Nietzche!