s. But it happens that
a man is healed or becomes ill, by one act. Therefore one act can
cause a habit.

_On the contrary,_ The Philosopher (Ethic. i, 7): "As neither does
one swallow nor one day make spring: so neither does one day nor a
short time make a man blessed and happy." But "happiness is an
operation in respect of a habit of perfect virtue" (Ethic. i, 7, 10,
13). Therefore a habit of virtue, and for the same reason, other
habits, is not caused by one act.

_I answer that,_ As we have said already (A. 2), habit is caused by
act, because a passive power is moved by an active principle. But in
order that some quality be caused in that which is passive the active
principle must entirely overcome the passive. Whence we see that
because fire cannot at once overcome the combustible, it does not
enkindle at once; but it gradually expels contrary dispositions, so
that by overcoming it entirely, it may impress its likeness on it.
Now it is clear that the active principle which is reason, cannot
entirely overcome the appetitive power in one act: because the
appetitive power is inclined variously, and to many things; while the
reason judges in a single act, what should be willed in regard to
various aspects and circumstances. Wherefore the appetitive power is
not thereby entirely overcome, so as to be inclined like nature to
the same thing, in the majority of cases; which inclination belongs
to the habit of virtue. Therefore a habit of virtue cannot be caused
by one act, but only by many.

But in the apprehensive powers, we must observe that there are two
passive principles: one is the _possible_ (See First Part, Q. 79, A.
2, ad 2) intellect itself; the other is the intellect which Aristotle
(De Anima iii, text. 20) calls "passive," and is the "particular
reason," that is the cogitative power, with memory and imagination.
With regard then to the former passive principle, it is possible for
a certain active principle to entirely overcome, by one act, the
power of its passive principle: thus one self-evident proposition
convinces the intellect, so that it gives a firm assent to the
conclusion, but a probable proposition cannot do this. Wherefore a
habit of opinion needs to be caused by many acts of the reason, even
on the part of the "possible" intellect: whereas a habit of science
can be caused by a single act of the reason, so far as the _possible_
intellect is concerned. But with regard to the lower apprehensive
pow