Thomas Aquinas

“ Yet because of the relationship of the extremes in all of them we can use the preposition "ex" [out of] , which denotes order; for we can truly and properly say that "being is made out of non-being," and "out of bread, the body of Christ," and "out of air, fire," and "out of white, black." But because in creation one of the extremes does not pass into the other, we cannot use the word "conversion" in creation, so as to say that "non-being is converted into being": we can, however, use the word in this sacrament, just as in natural transmutation. ”

“ For it is evident that every agent acts according as it is in act. But every created agent is limited in its act, as being of a determinate genus and species: and consequently the action of every created agent bears upon some determinate act. Now the determination of every thing in actual existence comes from its form. Consequently, no natural or created agent can act except by changing the form in something; and on this account every change made according to nature's laws is a formal change. ”

“ I answer that, It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh, and to drink blood. And therefore Christ's flesh and blood are set before us to be partaken of under the species of those things which are the more commonly used by men, namely, bread and wine. Secondly, lest this sacrament might be derided by unbelievers, if we were to eat our Lord under His own species. ”