To the third one proceeds thus. It seems that habitude can be generated through one act. For demonstration is an act of reason. But through one demonstration is caused the knowledge [scientiam] that is the habitude of one conclusion. Therefore habitudes is able to be caused from one act.
Further, just as one habitude happens to increase through multiplication, so an act happens to increase through intension. But acts being multiplied, habitude is generated. Therefore if one act is intensified a lot, it could be a generative cause of habitude.
Further, health and illness are sorts of habitude. But from one act a man happens to be healed or sickened. Therefore one act is able to cause habitude.
But contrariwise is what the Philosopher says in Ethic. I, that neither one swallow nor one day makes a spring, so certainly neither one day nor a short time makes beatitude or happiness. But beatitude is working according to a habitude of complete virtue, as is said in Ethic. I. Therefore the habitude, and for the same reason any other habitude, is not caused through one act.
I reply that it must be said that, just as has already been said, habitude is generated through act inasmuch as passive power is moved from some active principle. But in order for any quality to be caused in the passive, it is needful that the active wholly overcome the passive. Thus we see that because fire cannot at once overcome its combustible, it does not at once inflame it, but bit by bit casts down contrary dispositions so that, wholly overcoming it, it may impress its similitude on it. But it is manifest that the active principle that is reason, is not able wholly to overcome the appetitive power in one act because the appetitive power has itself in many ways and to many things; but through reason is judged, in one act, that something is sought [appetendum] according to determinate reasons and circumstances. Thus from this the appetitive power is not wholly overcome, so as to be brought mostly to the same thing, by the way of nature, as pertains to the habitude of virtue. And therefore the habitude of virtue is not able to through one act, but through many.
But in the apprehensive powers it must be considered that the passive is twofold, one of which is the possible intellect itself, but another intellect which Aristotle calls passive, which is particular reason, that is, the cogitative impulse along with the memorative and the imaginative. Therefore with respect to the first passive, there is able to be some active that by one act wholly overcomes the power of its passive, as one proposition known through itself [per se nota] convinces the intellect to assent firmly to a conclusion, which indeed a probable proposition does not do. Thus it is needful for opinionative habitude to be caused from many acts of reason, even on the part of the possible intellect, but habitude of knowledge [habitum scientiae] is possibly caused from one act of reason as regards teh possible intellect. But as regards inferior apprehensive impulses, it is necessary to reiterate the same act many times so that something may be impressed firmly on the memory. Thus the Philosophers in the book on memory and recollection says meditation confirms memory.
But bodily habitude is possibly caused from one act, if the active is of great force, as sometimes strong medicine at once induces health.
And from this is obvious the response to the objections.
[Thomas Aquinas, ST 2-1.51.3, my translation. The Dominican Fathers translation is here, the Latin is here.]
Thus we get a basic account of habituation, and also the first reason why it was necessary to determine that habitudes are qualities. Qualities admit of contraries, and thus when you induce a qualitative disposition in anything, you have to do so against any contrary disposition it might happen to have. Thus, where such contrary dispositions exist, you have to act enough to overcome the contrariety, whatever it may be, and depending on the situation, it may take many actions to do so. In the case of the intellect affecting appetitive powers, there need to be many actions, as also there will need to be many actions for the internal senses, and, depending on the situation, possibly the body.
But in purely intellectual matters that are certain, it is possible, as when understanding a proof at once gives you knowledge, for the habitude to arise from a single action. In other cases, as in probable matters, it takes many actions. (St. Thomas elsewhere characterizes opinion as arising from when we have reasons on both sides, but greater on one side, so this is again a case of action overcoming contrary disposition.)
