What exactly is it? It is unfortunate that Socrates did not ask Meno, because then we would probably have a definition by now.

Let's define an idea as "an expression in language". Then, a statement is philosophical, if it is an idea about another idea, i.e. a statement about another statement.

Why do I believe this?

If we look at the Platonic definition for knowledge, a justified (?true?) belief, we can see that knowledge is a statement (Q) that necessarily follows from another statement (Q), its justification:

P => Q

Hence, knowledge (Q) is a statement about another statement (P), which satisfies the restriction imposed by the arrow: "follows necessarily from".

This restriction is not necessarily satisfied in such arrow.

Still, if we wish to link both, even if we cannot necessarily argue the necessity of the arrow, we have general philosophy. In that sense, general philosophy is closely related to knowledge, but is not necessarily knowledge.

Knowledge statements form a subset of the philosophy set of statements. In knowledge, as opposed to general philosophy, we have successfully argued the necessity of the arrow.

If we have managed to link both statements using an epistemically accredited justification method, then the statement belongs in the epistemically-restricted knowledge subset of which that justification method is the set-theoretical membership function.

For example, if we manage to axiomatically derive a claim from the construction logic of its Platonic world, the claim is mathematical. If we manage to demonstrate that there are no counterexamples in a given set of real-world experimental test results, then the claim is scientific. If we have one or more witness depositions for the statement, then it is historical.

We can therefore define epistemology as the study and description of justification methods.

General philosophy, itself, is hypothetical knowledge for which we would possibly like to find a proper home in an epistemically-restricted subset of knowledge.

This may explain why modern knowledge disciplines originally used to be some form of philosophy.

For example, we no longer consider physics to be philosophy, but historically it used to be. In fact, physics is still philosophy today, but belonging to one of its epistemically-restricted subsets, i.e. science.

So, then we come to a set-theoretical question: Do the epistemically-restricted subsets of knowledge form a proper partition? In my opinion, by design, they do not overlap. However, it is much harder to argue that the union of these subsets also covers the complete set (of knowledge).

The question here is not whether we have discovered all knowledge (which we obviously haven't), but if we have successfully established the complete inventory of knowledge-justification methods? According to Immanuel Kant's Critique of Pure Reason, we did.