Even though the purpose of science is to identify truths, its immediate objective must be to model data, since it is only by modeling data that theoretical truths can be discovered. It follows that even though biological entities are composed of microphysical entities, biological truths are not microphysical truths. It also follows that psychological truths are not biological truths. But, so it is also shown, the sense in which psychology is non-identical with biology is stronger than the sense in which biology is non-identical with microphysics: for whereas any given biological datum is non-anomalous with respect to the microphysical realm, any given psychological datum is thus anomalous. Though intuitively obvious, this truth is hard to prove. But the present work proves it and in so doing provides a clear description of the structure of scientific theories. This brisk monograph rigorous proves that each scientific domain must be understood on its own terms and that any attempt to reduce one discipline to another is doomed from the start.