It is not easy to say what metaphysics is. Ancient and Medieval philosophers might have said that metaphysics was, like chemistry or astrology, to be defined by its subject-matter: metaphysics was the “science” that studied “being as such” or “the first causes of things” or “things that do not change”. It is no longer possible to define metaphysics that way, for two reasons. First, a philosopher who denied the existence of those things that had once been seen as constituting the subject-matter of metaphysics—first causes or unchanging things—would now be considered to be making thereby a metaphysical assertion. Second, there are many philosophical problems that are now considered to be metaphysical problems (or at least partly metaphysical problems) that are in no way related to first causes or unchanging things—the problem of free will, for example, or the problem of the mental and the physical.