[j1vms]

To go a bit further, everything as one perceives it is necessarily in one's mind (or nervous system, to be precise). One consciously or unconsciously determines that something is happening outside of their mind, which makes that determination simply a philosophical one. This is a reminder to all, as to why the highest degree granted at any conventional institution is labelled a Doctorate of Philosophy in a given subject.

I would go as far as to say that just as the Uncertainty Principle prescribes a limit on what is knowable in quantum physics, so does philosophy suggest limits on what will ever be rationally proven through human perception, ideas, and knowledge.