The problem is that any intellect needs some metaphysical basis to operate. On the one hand there are some a priori assumptions, like "there is an objective reality", "I can rely on my memory to some extent" etc. You should believe, otherwise you will be unable even to start thinking. On the other hand there are some high level generalizations which can never be proved. For example. Ramil says:
This is pure metaphysics but this is used indeed in actual natural science as a first approach. Even natural science can not avoid metaphysical generalizations.The fundamental law -- the law of conservation says that nothing can be spun out of nothingness.