This question seems either trivial or somewhat vague; let me explain further.
I apologize if I am misunderstanding the concepts or missing the point entirely; I am a mathematics student and I certainly lack any serious knowledge of philosophy. Still, I am interested if there is an opinion on this:
The terms "a priori" and "a posteriori" may roughly be identified with knowledge (or justification, rather) that is free from experience, in the first case, and contingent on experience, in the second case.
In mathematics one often finds the "a priori" term used in proof-writing, which I suppose may often just be for the sake of exclamation (somewhat like the use of "a fortiori") or even just for style points. I wonder whether there is merit to using the term in a setting (the mathematical one) in which all knowledge and justification is "obviously" a priori. Am I missing the point here?
On the other hand one find also the term "a posteriori" used in mathematical proofs. Let me give two examples:
(i) Suppose one wants to give a proof that a class of objects C has property P. It is not obvious that every object belonging to C has P, but after some pages of deductions we arrive at the conclusion that, indeed, any such object from C has P. We have only after close observation have found this, but may this justification be considered "a posteriori"? (I have seen the term used in this way.)
(ii) Assume now there is a property P that is satisfied by some, but not all objects of C. One sometimes finds statements like "It is not a priori true that A in C has P, but after fixing A we find, a posteriori, that A has P."
(As an example for those comfortable with some linear algebra: "It is not a apriori true that a k-vectorspace is finite-dimensional. The k-vectorspace of polynomials of degree n is a posteriori finite dimensional.")
I guess the two cases are similar. Would someone explain to me, whether or not this is a gross misuse of language or whether one might consider observations inside of mathematics as some kind of a posteriori knowledge?
Thanks a lot, I am happy to try to clarify further if this was unclear!
Edit: J.-P. Serre, one of the most renowned mathematicians ever and someone who is generally considered a great expositor, is a frequent user of the term "a priori". To give another example, consider this example from his "Local Fields" [p.79, Prop. 17]:
"[...]We know a priori that G(K_n/K) can be identified with a subgroup of G(n) (cf. Bourbaki, Alg., Chap. V, §11);[...]"
The statement is that some object (a Galois group) maybe identified with another, then he cites a source for a proof of the statement.
Another Edit: This is from Hartshorne's Algebraic Geometry:
"[...]We will begin our study in an oblique manner by defining the notion of an "abstract nonsingular curve" associated with a given function field. It will not be clear a priori that this is a variety. However, we will see in retrospect that we have defined nothing new.[...]"