I answer that, The difference between a counsel and a commandment is that a commandment implies obligation, whereas a counsel is left to the option of the one to whom it is given. Consequently in the New Law, which is the law of liberty, counsels are added to the commandments, and not in the Old Law, which is the law of bondage. We must therefore understand the commandments of the New Law to have been given about matters that are necessary to gain the end of eternal bliss, to which end the New Law brings us forthwith: but that the counsels are about matters that render the gaining of this end more assured and expeditious.
Now man is placed between the things of this world, and spiritual goods wherein eternal happiness consists: so that the more he cleaves to the one, the more he withdraws from the other, and conversely. Wherefore he that cleaves wholly to the things of this world, so as to make them his end, and to look upon them as the reason and rule of all he does, falls away altogether from spiritual goods. Hence this disorder is removed by the commandments. Nevertheless, for man to gain the end aforesaid, he does not need to renounce the things of the world altogether: since he can, while using the things of this world, attain to eternal happiness, provided he does not place his end in them: but he will attain more speedily thereto by giving up the goods of this world entirely: wherefore the evangelical counsels are given for this purpose.
Now the goods of this world which come into use in human life, consist in three things: viz. in external wealth pertaining to the "concupiscence of the eyes"; carnal pleasures pertaining to the "concupiscence of the flesh"; and honors, which pertain to the "pride of life," according to 1 Jn. 2:16: and it is in renouncing these altogether, as far as possible, that the evangelical counsels consist. Moreover, every form of the religious life that professes the state of perfection is based on these three: since riches are renounced by poverty; carnal pleasures by perpetual chastity; and the pride of life by the bondage of obedience.
Now if a man observe these absolutely, this is in accordance with the counsels as they stand. But if a man observe any one of them in a particular case, this is taking that counsel in a restricted sense, namely, as applying to that particular case. For instance, when anyone gives an alms to a poor man, not being bound so to do, he follows the counsels in that particular case. In like manner, when a man for some fixed time refrains from carnal pleasures that he may give himself to prayer, he follows the counsel for that particular time. And again, when a man follows not his will as to some deed which he might do lawfully, he follows the counsel in that particular case: for instance, if he do good to his enemies when he is not bound to, or if he forgive an injury of which he might justly seek to be avenged. In this way, too, all particular counsels may be reduced to these three general and perfect counsels.