Hence the scholastic aphorism regarding substantial change: _Corruptio unius est generatio alterius_: the corruption or destruction of one kind of material thing involves the generation of another kind. The concepts of _materia prima_ and _forma substantialis_ are concepts not of phenomenal entities directly accessible to the senses or the imagination, but of principles which can be reached only mediately and by intellect proper. They cannot be pictured in the imagination, which can only attain to the sensible. We may help ourselves to grasp them intellectually by the analogy of the shapeless block of marble and the figure educed therefrom by the sculptor, but this is only an analogy: just as the statue results from the union of an _accidental_ form with an existing matter, so this matter itself, the substance _marble_, is composed of a _substantial_ form and a primordial, _potential_ matter. But there the analogy ceases. Furthermore, when we consider that the proper and primary objects of the human intellect itself are corporeal things or bodies, and that these bodies actually exist in nature only as composite substances, subject to essential or substantial change, we shall realize why it is that the concept of _materia prima_ especially, being a mediate and negative concept, is so difficult to grasp; for, as the scholastics describe it, translating Aristotle’s formula, it is in itself _neque quid, neque quantum, neque quale, neque aliquid eorum quibus ens determinatur_.(81) But it is through intellectual concepts alone, and not through imagination images, that we may hope to analyse the nature and processes even of the world of corporeal reality; and, as St. Thomas well observes, it was because the ancient Greek atomists did not rise above the level of thinking in imagination images that they failed to recognize the existence, or explain the nature, of substantial change in the material universe(82): an observation which applies with equal force to those scientists and philosophers of our own time who would fain reduce all physical processes to mere mechanical change.