Theologicae regulae

Everything impossible according to a lower cause, from which follows something impossible according to a higher cause, is impossible to God.

That is called impossible according to a lower cause which cannot happen according to the accustomed course of nature, such as for a virgin to bear a child. Impossible according to a higher cause, however, is that which the higher cause, that is, God, cannot do, such as that God should not be good: for God cannot do this, that he should not be good; for this would be to derogate from the divine majesty. And thus everything impossible according to a higher cause is impossible according to a lower one, but not the reverse. For a stump to be a calf is impossible according to the law of nature; yet God could remove the nature of a stump from the subject, and impress upon it the nature of a calf: which, if it were done, the stump would cease to be a stump, and would begin to be a calf: although this happening is against nature, yet once done, what was previously naturally a stump would now, against nature, be a calf. But that a white thing, remaining white, should be black, is impossible not only according to a lower cause, but according to a higher; for although God could, in respect of substance, alter matter, so that wine be made from water, yet he cannot do this: that, since it is singularly water, it be wine; since it is white, it be black; since it is, it not be: for if it were white while it was black, God, by his own authority, would make it not be what he had, by his own authority, made to be: therefore he would be at odds with himself: which is impossible. And thus everything impossible according to a lower cause, from which follows something impossible according to a higher, is impossible to God. Similarly can be understood what is said by Jerome: Although God can do all things, he cannot make a virgin out of a corrupted woman; that is, God cannot make that someone be, at the same time, both corrupted and a virgin. But it is asked, since the natures of Creator and creature differ more than white and black (for these agree in no genus at all; white and black, however, agree in a subaltern genus), how could God make it so that God, being God, should be man, and could not make it so that white, being white, should be black. To which it is answered that a thing begins to be something, or of some kind, in two ways: either by mere concatenation, or by the union of another thing to itself. For example: something begins to be white by the concatenation of substance and a property, which is, for the subject, the cause of the predicate, according to which it begins to be white. God, however, begins to be man from the fact that human nature is united to the divine nature. Thus, then, God was able to make it so that, remaining God, he should be man, namely, by the union of human nature. But this he could not do by mere concatenation, so as to compose the humanity of a divine person, and not a man, that is, so as to unite body and soul to the person. Similarly he could have made a white thing, remaining white, to be black: if, by an ineffable union, according to which he united human nature to the divine, he should unite black to white, so that, just as it is said, God is man, it could truly be said: White is black: but this could not happen by mere concatenation, or union of white and black, so that to a white thing remaining white, blackness be impressed, and not by the grace of a black thing which is united to white.