I answer that, on this point the philosophers erred and certain modern heretics err. For some of the philosophers allowed that souls separated from bodies are reunited to bodies, yet they erred in this in two ways. First, as to the mode of reunion, for some held the separated soul to be naturally reunited to a body by the way of generation. Secondly, as to the body to which it was reunited, for they held that this second union was not with the selfsame body that was laid aside in death, but with another, sometimes of the same, sometimes of a different species. Of a different species when the soul while existing in the body had led a life contrary to the ordering of reason: wherefore it passed after death from the body of a man into the body of some other animal to whose manner of living it had conformed in this life, for instance into the body of a dog on account of lust, into the body of a lion on account of robbery and violence, and so forth---and into a body of the same species when the soul has led a good life in the body, and having after death experienced some happiness, after some centuries began to wish to return to the body; and thus it was reunited to a human body.
This opinion arises from two false sources. The first of these is that they said that the soul is not united to the body essentially as form to matter, but only accidentally, as mover to the thing moved, [*Cf. FP, Question [76], Article [1]] or as a man to his clothes. Hence it was possible for them to maintain that the soul pre-existed before being infused into the body begotten of natural generation, as also that it is united to various bodies. The second is that they held intellect not to differ from sense except accidentally, so that man would be said to surpass other animals in intelligence, because the sensitive power is more acute in him on account of the excellence of his bodily complexion; and hence it was possible for them to assert that man's soul passes into the soul of a brute animal, especially when the human soul has been habituated to brutish actions. But these two sources are refuted by the Philosopher (De Anima ii, 1), and in consequence of these being refuted, it is clear that the above opinion is false.
In like manner the errors of certain heretics are refuted. Some of them fell into the aforesaid opinions of the philosophers: while others held that souls are reunited to heavenly bodies, or again to bodies subtle as the wind, as Gregory relates of a certain Bishop of Constantinople, in his exposition of Job 19:26, "In my flesh I shall see my God," etc. Moreover these same errors of heretics may be refuted by the fact that they are prejudicial to the truth of resurrection as witnessed to by Holy Writ. For we cannot call it resurrection unless the soul return to the same body, since resurrection is a second rising, and the same thing rises that falls: wherefore resurrection regards the body which after death falls rather than the soul which after death lives. And consequently if it be not the same body which the soul resumes, it will not be a resurrection, but rather the assuming of a new body.