I'm a bit confused about how Universal Instantiation works. I read that you shouldn't really plug in any value "a" for x in "For all x P(x)" unless a choice for "a" pops up in the givens but in the following proof they just select any value y. Is that allowed? Or am I misunderstanding it and they got the Y from something else. If possible could you give me a detailed scratch-work of givens and goals.
Thanks.
The Question
Answer
