Meanwhile we must guard against conceiving the potential or material factor in change as a sort of actual but hidden core of reality which itself persists unchanged throughout; and the formative or actualizing factors as superficially adorning this substratum by constantly replacing one another. Such a substitution of imagination images for intellectual thought will not help, but rather hinder, all accurate analysis. It is not the potential or material factor in things that changes, nor yet the actualizing or formal factors, but the things themselves; and if “things” are subject to “real change” it is manifest that this fact can be made intelligible, if at all, only by intellectually analysing the things and their changes into constitutive principles or factors which are nor themselves “things” or “changes”. Were we to arrive only at principles of the latter sort, so far from explaining anything we would really only have pushed back the problem a step farther. It may be that none of the attempts yet made by philosophers or scientists to offer an ultimate explanation of change is entirely satisfactory,—the scholastic explanation will be gradually outlined in these pages,—but it will be of advantage at least to recognize the shortcomings of theories that are certainly inadequate. We are now in a position to state and explain the important scholastic aphorism embodying what has been called the Principle of Change (“_Principium Motus_”): _Quidquid movetur, ab alio movetur_: “Whatever undergoes change is changed by something else”. The term _motus_ is here taken in the wide sense of any real transition from potentiality to actuality, as is evident from the alternative statements of the same principle: _Nihil potest seipsum reducere e potentia in actum_: “Nothing can reduce itself from potentiality to actuality,” or, again, _Potentia, qua talis, nequit per semetipsam ad actum reduci, sed reducitur ab alio principio in actu_: “The potential as such cannot be reduced by itself to the actual, but only by some other already actual principle”.(77) This assertion, rightly understood, is self-evidently true; for the state of passive potentiality, as such, involves the absence of the correlative actuality in the potential subject; and since the actual, as such, involves a perfection which is not in the potential, the latter cannot confer upon itself this perfection: nothing can be the adequate principle or source of a perfection which is not in this principle or source: _nemo dat quod non habet_.