Intermixing of philosophical and metaphysical doctrines with empirical science poses an important problem, since such doctrines are often presented as scientific theories, which finally adulterates science itself. It is necessary to determine the criteria which distinguish science and metaphysical assumptions and therefore to evaluate the real essence and character of hypotheses and theories proposed as scientific ones. The problem appears in the explanation and interpretation of natural events. If the explanation offered is subject to a scientific test, then it is scientific. Otherwise, it is a philosophical or metaphysical assumption which cannot give any scientific knowledge. Empirical scientific testability is the only basis to determine and separate scientific theories from metaphysical adoptions.