It will have validity in the sense that if the theory is original AND you get published so your theory can be found by strangers, then somebody doing diligent research should find you as the earliest author.
That said, the world is unfair. In the realm of mathematics, which we should think would be logical and rational, there are dozens of famous theories invented by mathematician A but named after mathematicians B, that DID NOT invent them, sometimes even when B never attempted to take credit.
Primarily this is due to the fact that B did more with the theory, or presented it more formally AS a theory with better logic and proof, or it was in B's writing that real world consequences and applications for the theory arose. In other words, B did the work to make the theory famous or important, so the community of people working with such theories refer to it a B's theory, not the A's theory.
(I should also note there are cases where a female A has done all the work of the theory, and a male B supervisor has gotten all the credit for it; I believe there are a few such cases in early Astronomy.)
In a work of fiction, it will be difficult to present a philosophy that is argued like a person with a PhD in Philosophy would argue it. (Perhaps as an addendum in the back of the book you could). Such things contain copious references to prior philosophical work, careful explanations when the new philosophy runs counter to existing philosophical ideas and touchstones, and like all fields of advanced study, they have their own 'language' of terminology and reference they use as shorthand for others that already understand this.
In general, the practitioners of a field in which one can receive a PhD do not tolerate amateurs, simply because amateurs do not know the most basic rules of their game and how to present a new idea. Physics is rife with this, people that think they have an idea about how the universe works that is laughably naive and violates known physics in the first paragraph.
So even if your theory became well known, it would probably be forever attached to the real-life philosopher (B) that translated your musings into a formal philosophy, or at least made it famous. Leibniz was the first to publish and most likely invented most of the calculus we use today, but Newton gets the credit for it, because Newton was world famous and insisted he did it first.
Or your theory might get a generic name (e.g. Integration by Parts is a theory invented and proven by Brook Taylor, but is not named after him; although the Taylor Series he invented is named after him.)
Finally, copyright does not exactly protect an idea, it protects the words used to express that idea. If somebody takes your philosophical ideas and is able to express it in their own words with their own invented terminology, they may not violate your copyright at all.
For example, the idea of a Wizard's School in a magical place is not a protected idea, even though that idea is central to the Harry Potter series.
In fact Lev Grossman had the same idea, published in 2009, and his work is the basis of the recent TV series "The Magicians"; still running. It is far more adult fare with adult students (more like college than grade school), including numerous sex scenes and sexual references, but the basic idea that students attend classes and learn magic to solve their life-threatening problems is there.