Theoretical entities in modern science can be hard to represent to ourselves due to the limitations of our conceptions to perceptible forms and properties.

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