What does a prime (apostrophe) mean before a predicate? I found this statement in a paper by John McCarthy:
$$
\forall x.ostrich\ x \supset  species\ x ={}^\prime{}ostrich
$$
I can't figure out what the prime indicates.

An apostrophe before the name of a predicate has no standard meaning. I believe McCarthy is just using it as a systematic way for assigning symbolic identifiers to classes, so that he can economise on axioms. I.e., if instead of saying $ostrich(x)$ you say $species(x) = 452$ and instead of saying $penguin(x)$ you say $species(x) = 453$, then you avoid the possibility of having an $x$ that satisfies $ostrich(x) \land penguin(x)$. $'ostrich$ and $'penguin$ are McCarthy's convention for naming the symbolic identifiers, rather than using specific identifiers like $452$ and $453$.
