  Even at this, unobservables should not be specialized natures or forces taken to account just for certain phenomena. This was, according to Wright, the problem with scholastic substantial forms (PD 166–67). His criticism of metaphysical concepts was that they are empirically poor; they do not link different phenomena and do not generate predictions that can be verified at the level of the tangible and visible. Unlike early modern critics of scholastic metaphysical concepts, Wright did not claim that scientific concepts are by comparison clear and simple.  Indeed, theoretical entities in modern science can be hard to represent to ourselves because of the limitations of our conceptions to perceptible forms and properties (PD 166). Wright speculated that there were “orders of forces” between the physico-chemical and the vital, just as there are intermediate phenomena between the vegetative functions of an animal and sensibility, i.e., sensation and perception. But since sensibility presents the elements from which conceptions of size and movement must come, our conceptions of forces and hidden elements are limited to the sensible (PD 167). There are thus areas of nature we would investigate that are largely inaccessible to us because of empirical limitations. Wright did not resort to reductionism to bridge this gap in our knowledge. He said, “Can sensibility and the movements governed by it be derived directly by chemical synthesis from the forces of inorganic elements? It is probable, both from analogy and direct observation, that they cannot” (PD 167). To determine what theoretical entities are real is difficult but is nevertheless the task of science, which always concerns itself with  facts.[5]