It would seem that nothing remains to be investigated about this subject; for, when we have reached the _first_ constituent principles of a given essence, the metaphysical analysis is at an end. One question, however, remains to be settled between us and the philosophers of the Aristotelic school concerning the _mutual relation_ of the matter and the substantial form in a material being. Is such a relation variable or invariable? Is the matter separable from any given substantial form, as the Aristotelic theory assumes, or are the matter and its form so bound together as to form a unit substantially unchangeable? Can substantial forms be supplanted and superseded by other substantial forms, or do they continue for ever as they were at the instant of their creation? Some of our readers may think that what we have said in other preceding articles suffices to settle the question; for it is obvious that simple material elements are substantially unchangeable. But the peripatetic school looked at things from a different point of view, and thought that the question was to be solved by the consideration of the potentiality of the _first matter_ with respect to all substantial forms. Hence it is under this aspect that their opinion is to be examined, that a correct judgment may be formed of the merits and deficiencies of a system so long advocated by the most celebrated philosophers. For this reason, as also because some modern writers have resuscitated this system without taking notice of its defects, and without making such corrections as were required to make it agree with the positive sciences, we think it necessary to examine the notions on which the whole Aristotelic theory is established, and the reasonings by which it is supported, and to point out the inaccuracies by which some of those reasonings are spoiled, as well as the limits within which the conclusions of the school can be maintained. _Materia prima._--The notion of “first matter,” which plays the principal part in the theory of substantial generations, has been the source of many disputes among philosophers. Some, as Suarez, think that the _materia prima_ is metaphysically constituted of act and potency; others consider the _materia prima_ as a real potency only; whilst others consider it as a mere potency of being, and therefore as a non-entity. The word “matter” can, in fact, be used in three different senses. First, it is used for _material substance_, either compound or simple; as when steel is said to be the matter of a sword, or when the primitive elements are said to be the matter of a body. When taken in this sense, the word “matter” means a _physical_ being, substantially perfect, and capable of accidental modifications.