Other basic problems with regard to matter and force filled the minds of the medieval schoolmen quite as they do those of the modern generations. For instance, they occupied themselves with the question of the indestructibility of matter, and also, strange as it may seem, with the conservation of energy. We have presumably learned so much by experimental demonstration and original observation in the physical sciences in the modern times, and especially during the precious nineteenth century, that any thinking of the medieval mind along these lines might, in the opinion of those who know nothing of what they speak, be at once set aside without further question as preposterous, or at best nugatory. The opinions of medieval scholars in these matters would be presumed, without more ado, to have been so entirely speculative as to deserve no further attention. Nothing could well be farther from the truth than this. Nowhere will more marvelous anticipations of what is most modern in science be found than in some of these considerations of basic principles in the physical sciences.
For instance, Thomas Aquinas, usually known as St. Thomas, in a series of lectures given at the University of Paris toward the end of the third quarter of the thirteenth century, stated as the most important conclusion with regard to matter that “Nihil omnino in nihilum redigetur.–Nothing at all will ever be reduced to nothingness.” By this, as is very evident from the context, he meant to say that matter would never be annihilated and could never be destroyed. It might be changed in various ways, but it could never go back into the nothingness from which it had been taken by the creative act. Annihilation was pronounced as not being a part of the scheme of things as far as the human mind could hope to fathom its meaning.
In this sentence, then, Thomas of Aquin was proclaiming the doctrine of the indestructibility of matter. It was not until well on in the nineteenth century that the chemists and physicists of modern times realized the truth of this great principle.
James J. Walsh, The Popes and Science (1908)