the active and
the passive principle of its act, as we see in human acts. For the
acts of the appetitive power proceed from that same power according
as it is moved by the apprehensive power presenting the object: and
further, the intellective power, according as it reasons about
conclusions, has, as it were, an active principle in a self-evident
proposition. Wherefore by such acts habits can be caused in their
agents; not indeed with regard to the first active principle, but
with regard to that principle of the act, which principle is a mover
moved. For everything that is passive and moved by another, is
disposed by the action of the agent; wherefore if the acts be
multiplied a certain quality is formed in the power which is passive
and moved, which quality is called a habit: just as the habits of
moral virtue are caused in the appetitive powers, according as they
are moved by the reason, and as the habits of science are caused in
the intellect, according as it is moved by first propositions.

Reply Obj. 1: The agent, as agent, does not receive anything. But
in so far as it moves through being moved by another, it receives
something from that which moves it: and thus is a habit caused.

Reply Obj. 2: The same thing, and in the same respect, cannot be
mover and moved; but nothing prevents a thing from being moved by
itself as to different respects, as is proved in Physics viii, text.
28, 29.

Reply Obj. 3: The act which precedes the habit, in so far as
it comes from an active principle, proceeds from a more excellent
principle than is the habit caused thereby: just as the reason is a
more excellent principle than the habit of moral virtue produced in
the appetitive power by repeated acts, and as the understanding of
first principles is a more excellent principle than the science of
conclusions.
________________________

THIRD ARTICLE [I-II, Q. 51, Art. 3]

Whether a Habit Can Be Caused by One Act?

Objection 1: It would seem that a habit can be caused by one act. For
demonstration is an act of reason. But science, which is the habit of
one conclusion, is caused by one demonstration. Therefore habit can
be caused by one act.

Obj. 2: Further, as acts happen to increase by multiplication so do
they happen to increase by intensity. But a habit is caused by
multiplication of acts. Therefore also if an act be very intense, it
can be the generating cause of a habit.

Obj. 3: Further, health and sickness are habit