Why is the word "anything" used even though it cannot be used for affirmative sentences? Why isn't it "something"? I would say it like “It is only a fool who becomes something.”
The reason that anything works here is because, although the sentence is technically positive polarity, it has a type of negative meaning.
Words like anything, ever, at all, and yet usually occur in negative sentences:
- *I have got anything yet. (ungrammatical)
- I haven't got anything yet.
- *I have ever been to Paris. (ungrammatical)
- I haven't ever been to Paris.
However, they can also occur in other sentences which have some type of negative meaning. For example, yes/no questions represent a choice between a positive and negative version of the proposition. So these work fine in yes/no questions:
- Have you got anything yet?
- Have you ever been to Paris?
These types of word are often known as ɴᴇɢᴀᴛɪᴠᴇ ᴘᴏʟᴀʀɪᴛʏ ɪᴛᴇᴍs. The environments they occur in are sometimes known as ɴᴇɢᴀᴛɪᴠᴇ ᴛʀɪɢɢᴇʀs.
The word only, which we see in the Original Poster's example, has a type of hidden negative meaning. Consider:
- Only Usain Bolt has run the 100m in under 9.6 seconds.
This sentence means the same as:
- Nobody has run the 100m in under 9.6 seconds, apart from Usain Bolt.
And because of the negative meaning involved, we can use negative polarity items such as ever here:
- Only Usain Bolt has ever run the 100m in under 9.6 seconds.
The word only is therefore often a negative trigger, allowing the use of words such as any, every and so forth.
The Original Poster's sentence is:
- It is only the fool who becomes anything.
This means the same as:
- Nobody becomes anything, apart from the fool.
This hidden negative meaning makes the use of anything possible here. Notice we could easily use other negative polarity items here too, for example ever or at all:
- It is only the fool who ever becomes anything at all.