We proceed to the first article thus:

1. It seems that original sin is not a habit. As Anselm says (De Conceptu Virginali 2, 3, 26), original sin is the lack of original justice. It is therefore a kind of privation. But a privation is opposed to a habit. Hence original sin is not a habit.

2. Again, the character of guilt attaches to actual sin more than to original sin, since actual sin has more of the nature of the voluntary. But there is no guilt in the habit of actual sin. If there were, a man would sin guiltily while he slept. There cannot then be any guilt in a habit which is original.

3. Again, an act of sin always precedes the habit of it, because sinful habits are always acquired, never infused. But there is no act which precedes original sin. Hence original sin is not a habit.

On the other hand: Augustine says (De Baptismo Puer; De Peccat. Mer. et Remis. I, ch.39; De Tempt., Sermo 45): "because of original sin infants have a tendency to desire, even though they do not actually desire." Now we speak of a tendency where there is a habit. Original sin is therefore a habit.

I answer: as we said in Q.50, Art.1, there are two kinds of habit. [27] There is the habit which inclines a power to act, of the kind which enables us to say that sciences and virtues are habits. Original sin is not a habit of this kind. But we also give the name of habit to the disposition by which a composite nature is well or ill disposed in a certain way, especially when such a disposition has become almost second nature, as in the case of sickness or of health. Original sin is such a habit. It is the disordered disposition which has resulted from the dissolution of the harmony which was once the essence of original justice, just as bodily sickness is the disordered disposition of a body which has lost the equilibrium which is the essence of health. Original sin is accordingly called the languor of nature.

On the first point: just as sickness of the body involves positive disorder in the disposition of the humours, as well as privation of the equilibrium of health, so original sin involves disorder in the disposition of the parts of the soul, as well as the privation of original justice. It is more than mere privation. It is a corrupt habit.

On the second point: actual sin is the disorder of an act. But original sin is the disordered disposition of nature itself, since it is the sin of nature. Now this disordered disposition has the character of guilt in so far as it is inherited from our first parent, as we said in Q.81, Art. i. It also has the character of a habit, which the disordered disposition of an act has not. Original sin can therefore be a habit, though actual sin cannot be a habit.

On the third point: this objection argues about the kind of habit which inclines a power to act. Original sin is not a habit of this kind, although it does result in an inclination to disordered actions. It results in such inclination not directly but indirectly, through depriving us of the original justice which would have prevented disorderly actions, and once did prevent them. The inclination to disordered bodily functions results from sickness in this same indirect way. But we should not say that original sin is an infused habit, nor that it is acquired through action (unless the action of our first parent, but not that of any present person). It is inborn by reason of our corrupt origin.

[27] A habit is defined as "a disposition of a subject which is in a state of potentiality either in respect of form or in respect of operation," but is distinguished from a "disposition" as being difficult to change. See "The Role of Habitus in the Thomistic Metaphysics of Potency and Act" in Essays in Thomism, Ed. R. E. Brennan.