While reading the Wikipedia article on infinite sets I found the following quote:
A set is infinite if and only if for every natural number, the set has a subset whose cardinality is that natural number.
If the axiom of choice holds, then a set is infinite if and only if it includes a countable infinite subset.
This raised an interesting question; without AC, is it possible to have a set where, given any natural number, you can find a subset with cardinality great than that number, but not necessary one equal to it?
For example a set where all definable subsets have even cardinality?
Edit to clarify the question: are there sets that satisfy the intuitive meaning of infinat but not the definition as quoted above?
FWIW, id be interested in both the case where "subsets larger than n exist" requires those subsets be finite and where they don't. (Though I suspect the second case is uninteresting as most of the interesting properties would be trivially true.)
No, this is impossible.
If $A$ has a subset $B$ such that $B$ has size greater than $n$, then there is, by definition, an injective function $f\colon\{0,\dots,n-1\}\to B$, which is also an injective function to $A$. Taking the image of $f$ is a subset of $A$ with exactly $n$ elements.