When St. Thomas used the aphorism "Nothing at all will ever be reduced to nothingness" there was another signification that he attached to the words quite as clearly as that by which they expressed the indestructibility of matter. For him _Nihil_ or nothing meant neither _matter_ nor _form_, that is, neither the material substance nor the energy which is contained in it. He meant then, that no energy would ever be destroyed as well as no matter would ever be annihilated. He was teaching the conservation of energy as well as the indestructibility of matter. Here once more the experimental demonstration of the doctrine was delayed for over six centuries and a half. The truth itself, however, had been reached by this medieval master-mind and was the subject of his teaching to the university students in Paris in the Thirteenth Century. These examples should, I think, serve to illustrate that the minds of medieval students were occupied with practically the same questions as those which are now taught to the university students of our day. There are, however, some even {40} more striking anticipations of modern teaching that will serve to demonstrate this community of educational interests in spite of seven centuries of time separation. In recent years we have come to realize that matter is not the manifold material we were accustomed to think it when we accepted the hypothesis that there were some seventy odd different kinds of atoms, each one absolutely independent of any other and representing an ultimate term in science. The atomic theory from this standpoint has proved to be only a working hypothesis that was useful for a time, but that our physicists are now agreed must not be considered as something absolute. Radium has been observed changing into helium and the relations of atoms to one another as they are now known, make it almost certain that all of them have an underlying sub-stratum the same in all, but differentiated by the dynamic energies with which matter in its different forms is gifted. Sir Oliver Lodge has stated this theory of the constitution of matter very clearly in recent years, and in doing so has only been voicing the practically universal sentiment of those who have been following the latest developments in the physical sciences. Strange as it may appear, this was exactly the teaching of Aquinas and the schoolmen with regard to the constitution of matter. They said that the two constituting principles of matter were prime matter and form. By prime matter they meant the material sub-stratum the same in all material things. By form they meant the special dynamic energy which, entering into prime matter, causes it to act differently from other kinds and gives it all the particular qualities by which we recognize it. This theory was not original with them, having been adopted from Aristotle, but it was very clearly set forth, profoundly discussed, and amply illustrated by the schoolmen.