aterial things whose properties we perceive. Therefore, we are inexorably lead to the conclusion that in order for a physical theory to provide an explanation we must first understand the nature of material reality. This of course assumes that there is such a reality, distinct from appearances. However, this has now become a metaphysical question (the answer cannot come from empirical evidence since this deals only in sensation). So we are presented with a problem: if the value of a physical theory depends on the metaphysical system adopted then it is impossible for such a theory to achieve universal acceptance. There are many schools of metaphysics (Aristotelian, Newtonian, atomistic and Cartesian) and they all hold different ideas as to the nature of reality (Encarta Electronic Encyclopida). They are all at odds because each can find fault with properties of matter assumed by others. If their metaphysical beliefs are in conflict then by definition so would be the physical theories they adopt.
             Duhem goes on to say that no single metaphysical doctrine can be sufficient to create a physical theory. All principles used in a theory must be derived from the metaphysics; otherwise the theory cannot by necessity be true. However no metaphysics gives instruction exact enough or detailed enough to derive all the elements necessary for a physical theory. Our conclusion then is that physical theory as an explanation is not possible. We cannot agree on a universal metaphysics and even if
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