This “inherence” of accident in substance, this mode of being whereby it affects, determines or modifies the substance, differs from accident to accident; these, in fact, are classified into _suprema genera_ by reason of their different ways of affecting substance (60). To this we shall return later. Here we may inquire, about this general relation of accident to substance, whether it is _essential_ to an accident _actually to inhere in_ a substance, if not immediately, then at least through the medium of some other accident. We suggest this latter alternative because as we shall see presently there are some accidents, such as colour, taste, shape, which immediately affect the _extension_ of a body, and only through this the substance of the body itself. Now the ordinary course of nature never presents us with accidents except as inhering, mediately or immediately, in a substance. Nor is it probable that the natural light of our reason would ever suggest to us the possibility of an exception to this general law. But the Christian philosopher knows, from Divine Revelation, that in the Blessed Eucharist the _quantity_ or _extension_ of bread and wine, together with the taste, colour, form, etc., which affect this extension, _remain in existence_ after their connatural substance of bread and wine has disappeared by transubstantiation. In the supernatural order of His providence God preserves these accidents in existence without a subject; but in this state, though they do not _actually inhere_ in any substance, they _retain their natural aptitude and exigence_ for such inherence. The Christian philosopher, therefore, will not define accident as “the mode of being which inheres in a subject,” but as “the mode of being which _in the ordinary course of nature_ inheres in a subject,” or as “the mode of being which has _a natural exigence_ to inhere in a subject”. It is not _actual inherence_, but the _natural exigence to inhere_, that is essential to an accident as such.(250) Furthermore, an accident needs a substance not formally _qua_ substance, or as a mode of being naturally existing in itself; it needs a substance _as a subject_ in which to inhere, which it will in some way affect, determine, qualify; but the subject in which it immediately inheres need not always be a substance: it may be some other accident, in which case both of course will naturally require some substance as their ultimate basis. Comparing now the concept of accident with that of substance, we find that the latter is presupposed by the former; that the latter is prior _in thought_ to the former; that we conceive accident as something over and above, something superadded to substance as subject. For instance, we can define matter and form without the prior concept of body, or animality and rationality without the prior concept of man; but we cannot define colour without the prior concept of body, or the faculty of speech without the prior concept of man.(251)