I answer that, A form is said to be corrupted directly by its contrary; indirectly, through its subject being corrupted.  When therefore a habit has a corruptible subject, and a cause that has a contrary, it can be corrupted both ways.  This is clearly the case with bodily habits—­for instance, health and sickness.  But those habits that have an incorruptible subject, cannot be corrupted indirectly.  There are, however, some habits which, while residing chiefly in an incorruptible subject, reside nevertheless secondarily in a corruptible subject; such is the habit of science which is chiefly indeed in the “possible” intellect, but secondarily in the sensitive powers of apprehension, as stated above (Q. 50, A. 3, ad 3).  Consequently the habit of science cannot be corrupted indirectly, on the part of the “possible” intellect, but only on the part of the lower sensitive powers.

We must therefore inquire whether habits of this kind can be corrupted directly.  If then there be a habit having a contrary, either on the part of itself or on the part of its cause, it can be corrupted directly:  but if it has no contrary, it cannot be corrupted directly.  Now it is evident that an intelligible species residing in the “possible” intellect, has no contrary; nor can the active intellect, which is the cause of that species, have a contrary.  Wherefore if in the “possible” intellect there be a habit caused immediately by the active intellect, such a habit is incorruptible both directly and indirectly.  Such are the habits of the first principles, both speculative and practical, which cannot be corrupted by any forgetfulness or deception whatever:  even as the Philosopher says about prudence (Ethic. vi, 5) that “it cannot be lost by being forgotten.”  There is, however, in the “possible” intellect a habit caused by the reason, to wit, the habit of conclusions, which is called science, to the cause of which something may be contrary in two ways.  First, on the part of those very propositions which are the starting point of the reason:  for the assertion “Good is not good” is contrary to the assertion “Good is good” (Peri Herm. ii).  Secondly, on the part of the process of reasoning; forasmuch as a sophistical syllogism is contrary to a dialectic or demonstrative syllogism.  Wherefore it is clear that a false reason can corrupt the habit of a true opinion or even of science.  Hence the Philosopher, as stated above, says that “deception is the corruption of science.”  As to virtues, some of them are intellectual, residing in reason itself, as stated in Ethic. vi, 1:  and to these applies what we have said of science and opinion.  Some, however, viz. the moral virtues, are in the appetitive part of the soul; and the same may be said of the contrary vices.  Now the habits of the appetitive part are caused therein because it is natural to it to be moved by the reason.  Therefore a habit either of virtue or of vice, may be corrupted by a judgment of reason, whenever its motion is contrary to such vice or virtue, whether through ignorance, passion or deliberate choice.