Alwyas a bit risky trying to demarcate subjects, but one way to approach it is to think why do we do Philosophy? What prompts someone to ask "What is the philosophy of X?" (where X can be practically anything!)
To ask about the philosophy of X is different than to ask about the contents of X. For example, take X to be "mathematics".
Content of Mathematics consists of axioms, theorems, proofs, definitions etc.
The philosophy of mathematics consists of foundational questions:
- What is a number?
- What is a successful proof?
- Is mathematics created or discovered?
The American Philosophical Association has a breakdown of the subfields of Philosophy. We can see that it can be broken down into three "buckets"
1. General philosophy: (let's call these "perspectives")
- Logic: What is correct reasoning?
- Metaphysics: How many categories of things fundamentally exist and what are their properties?
- Ethics: How do we decide what we should do vs can do?
- Epistemology: How do we know things?
2. History of Philosophy:
3. Special philosophy: General philosophy applied to a specific topic.
I like to think of Philosophy as a matrix. You have columns being the general philosophical perspectives and the rows being domains/topic of application.
One domain is "General" to capture, say, foundational metaphysics that applies overall. Then we have an ever-growing list of topics of application.
In each case, we want to know something about the what are the elements we are discussing, how do we gain knowledge of them, how do we reason about them, and what are any ethical considerations.
So to me that is the purpose of philosophy. If I ever need to understand how the four core "Philosophical perspectives" apply to something, I am engaging in philosophy (even if it is being done by a non-philosopher, which is probably where 99% of philosophy happens....poorly lol)