We must observe that the conversion of bread into the Body of Christ falls under a different category from all natural conversions. In every natural conversion the subject remains, and in that subject different forms succeed one another: hence these are called 'formal conversions.' But in this conversion subject passes into subject, while the accidents remain: hence this conversion is termed 'substantial.' Now we have to consider how subject is changed into subject, a change which nature cannot effect. Every operation of nature presupposes matter, whereby subjects are individuated; hence nature cannot make this subject become that, as for instance, this finger that finger. But matter lies wholly under the power of God, since by that power it is brought into being: hence it may be brought about by divine power that one individual substance shall be converted into another pre-existing substance. By the power of a natural agent, the operation of which extends only to the producing of a change of form and presupposes the existence of the subject of change, this whole is converted into that whole with variation of species and form.* So by the divine power, which does not presuppose matter, but produces it, this matter is converted into that matter, and consequently this individual into that: for matter is the principle of individuation, as form is the principle of species.* Hence it is plain that in the change of the bread into
the Body of Christ there is no common subject abiding after the change, since the change takes place in the primary subject [i.e., in the matter], which is the principle of individuation. Yet something must remain to verify the words, This is my body, which are the words significant and effective of this conversion. But the substance does not remain: we must say therefore that what remains is something beside the substance, that is, the accident of bread. The accidents of bread then remain even after the conversion.
This then is one reason for the accident of bread remaining, that something may be found permanent under the conversion. Another reason is this. If the substance of bread was converted into the Body of Christ, and the accidents of bread also passed away, there would not ensue upon such conversion the being of the Body of Christ in substance where the bread was before: for nothing would be left to refer the Body of Christ to that place. But since the dimensions of bread (quantitas dimensiva panis), whereby the bread held this particular place, remain after conversion, while the substance of bread is changed into the Body of Christ, the Body of Christ comes to be under the dimensions of bread, and in a manner to occupy the place of the bread by means of the said dimensions.
4.61 : Of the Eucharist
4.64 : An Answer to Difficulties raised in respect of Place