A. About the First Doubt

369. As to the first doubt, it would seem prima facie that such bread would be consecratable, first because it is usually reckoned to be wheat bread and Christ instituted the sacrament in the usual wheat bread, and second because the purifications done in preparing starch do not seem to be alterations from the species of wheat flour (for only colorings and purifications and the like are done there, and these do not seem to alter the core of the wheat away from its species).

370. However, if one is to believe the experts in given subjects about the qualities consequent to those subjects - subjects that are made known through the effects by which we recognize qualities and functions and distinguish substances (for we do not recognize the distinction between fire and water otherwise than through what becomes known to us from their effects), and each species, according to medical doctors, gets from the whole species any quality and function that does not belong to it through such and such mixing but follows the form proper to such species, and this sort of quality and function is different in the whole species in starch and in wheat bread (as is plain to those who inspect them and assign them such properties as concerns their effects) - if all this is so, it seems that starch is of a totally different species from wheat bread.

371. And there is a confirmation: for the fact that vinegar and sour grape juice differ from wine in species, and are consequently not matter for consecration as to the wine, we do not otherwise prove than by reference to their qualities and functions, namely because vinegar is cold from its whole species and wine is warm from its whole species.

372. Here it can briefly be said that, since it is certain that wheat flour ground in the common way of grinding, namely in the way that millers are accustomed to grind it, is suitable matter for making consecratable bread, and since it is doubtful whether starch is matter of this sort, and the negative side is much more probable, not without sin would anyone expose oneself to such a doubt, especially he who prepares hosts for consecration, since the fact that he is preparing them from this sort of matter is simply certain to him. Perhaps the user can be more excused of sin, because he supposes that such matter has been well prepared. But if this matter is not in truth fit for consecration, he effects nothing [when consecrating it]. There is, however, no idolatry for the people or occasion of idolatry, since the people, as far as is in themselves, are supposing that the matter is suitable and is to be adored both by themselves and others.