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There are certainly no clear boundaries within science and philosophy. Science is about knowledge of nature, hypothesis, tests and repetition, and philosophy is about knowledge generated purely by reason, not objectively verifiable by the scientific method (hope providing a good approximation, if not, thanks in advance for any correction).
How do we know whether a discipline is science or philosophy?
For example, the systems theory seem to match both descriptions.
- A rock can be perfectly described by the systems theory from a scientific approach (e.g. we can test with the scientific method that rocks are systems because they are formed by parts holding persistent relations, for example, we can test that molecules joined by molecular bonds under X conditions form rocks); we can test it several times and prove that the systems theory provides a valid and testable hypothesis regarding a natural entity.
- Or, a rock can also be described from a philosophical approach by the same theory: we can state that as any other system, rocks tend to keep existence as a whole because their parts keep relations (there is no way to prove otherwise: only rocks formed by relations can exist, and the lack of relations between whatever parts can't be considered a rock).
This question is about a discipline, not about philosophy/science. There are clear differences between them, and the point is to assess whether a discipline can be considered to be mostly one or the other.