Objection 3. Further, Augustine says (De Civ. Dei xiv, 16) that "the movement of the genital members is sometimes inopportune and not desired; sometimes when sought it fails, and whereas the heart is warm with desire, the body remains cold." Therefore the movements of the members are not obedient to reason.
Reply to Objection 3. As Augustine says (De Civ. Dei xiv, 17,20) it is in punishment of sin that the movement of these members does not obey reason: in this sense, that the soul is punished for its rebellion against God, by the insubmission of that member whereby original sin is transmitted to posterity.
But because, as we shall state later on, the effect of the sin of our first parent was that his nature was left to itself, through the withdrawal of the supernatural gift which God had bestowed on man, we must consider the natural cause of this particular member's insubmission to reason. This is stated by Aristotle (De Causis Mot. Animal.) who says that "the movements of the heart and of the organs of generation are involuntary," and that the reason of this is as follows. These members are stirred at the occasion of some apprehension; in so far as the intellect and imagination represent such things as arouse the passions of the soul, of which passions these movements are a consequence. But they are not moved at the command of the reason or intellect, because these movements are conditioned by a certain natural change of heat and cold, which change is not subject to the command of reason. This is the case with these two organs in particular, because each is as it were a separate animal being, in so far as it is a principle of life; and the principle is virtually the whole. For the heart is the principle of the senses; and from the organ of generation proceeds the seminal virtue, which is virtually the entire animal. Consequently they have their proper movements naturally: because principles must needs be natural, as stated above (Reply to Objection 2).
INVOLUNTARY MOVEMENT OF THE MEMBERS