A materialist understands the principle of reality to be matter, with “reality” designating “the actual things which are now.” Materialism can be illustrated by considering the difference between an architect’s idea of a house, and the real house which is the result of that idea. The architect’s idea is “realized” in the wood and brick (i.e., the “matter”) of the structure. Hence, the simple difference between the real and the unreal is “matter.” An extreme materialist denies the reality of anything except matter, while a more moderate materialist allows for some non-material realities such as ideas. For even the moderate materialist, however, such non-material realities arise from matter and are completely dependent upon matter for their existence. For instance, in the philosophy of mind, an epiphenomenalist holds that physical states (i.e., brain and neural activity) are the cause of mental states (mind), but never the other way around. Accordingly, if the physical state ceases, the mental state also ceases, for the mental state is merely a “sign” of matter, much as a shadow is a “sign” of an object. In any regard, for all materialists, the things we see are material and the ultimate elements constitute the principle of reality. Thus, the materialist denies spiritual reality and explains life and consciousness in terms of the non-living and non-conscious.
In order for materialism to be true, reality must be identical with matter; the sensible must be the real without qualification. It is here that materialism encounters insuperable difficulties. As extreme materialists describe it, matter is always something determinable. We never experience matter as matter but always as some fully determinate kind of matter. At the same time, however, as the principle of reality, matter must also be fully determinate. In other words, materialism reduces to the self-contradictory assertion that the indeterminate is the fully determinate by which the determinable is made determinate. In addition to that, though we unquestionably have experience with material reality, we also have immediate experience of im material reality, namely, our own conscious activity. Our thoughts and choices do not present themselves as sensible things, yet they are just as real as sensible things. As a logical consequence of his basic principle, however, the materialist must affirm that such conscious activities are essentially indistinct from nonconscious activities, a notoriously inexplicable and seemingly counter-intuitive assertion. Thus, while materialism is attractive to many because of its direct appeal to sensible things, it ultimately fails as a metaphysical principle.