Objection 1. It would seem that certain definite counsels are not fittingly proposed in the New Law. For counsels are given about that which is expedient for an end, as we stated above, when treating of counsel (14, 2). But the same things are not expedient for all. Therefore certain definite counsels should not be proposed to all.
Objection 2. Further, counsels regard a greater good. But there are no definite degrees to the greater good. Therefore definite counsels should not be given.
Objection 3. Further, counsels pertain to the life of perfection. But obedience pertains to the life of perfection. Therefore it was unfitting that no counsel of obedience should be contained in the Gospel.
Objection 4. Further, many matters pertaining to the life of perfection are found among the commandments, as, for instance, "Love your enemies" (Matthew 5:44), and those precepts which Our Lord gave His apostles (Matthew 10). Therefore the counsels are unfittingly given in the New Law: both because they are not all mentioned; and because they are not distinguished from the commandments.
On the contrary, The counsels of a wise friend are of great use, according to Proverbs 27:9: "Ointment and perfumes rejoice the heart: and the good counsels of a friend rejoice the soul." But Christ is our wisest and greatest friend. Therefore His counsels are supremely useful and becoming.
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 John 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.
Reply to Objection 1. The aforesaid counsels, considered in themselves, are expedient to all; but owing to some people being ill-disposed, it happens that some of them are inexpedient, because their disposition is not inclined to such things. Hence Our Lord, in proposing the evangelical counsels, always makes mention of man's fitness for observing the counsels. For in giving the counsel of perpetual poverty (Matthew 19:21), He begins with the words: "If thou wilt be perfect," and then He adds: "Go, sell all [Vulgate: 'what'] thou hast." In like manner when He gave the counsel of perpetual chastity, saying (Matthew 19:12): "There are eunuchs who have made themselves eunuchs for the kingdom of heaven," He adds straightway: "He that can take, let him take it." And again, the Apostle (1 Corinthians 7:35), after giving the counsel of virginity, says: "And this I speak for your profit; not to cast a snare upon you."
Reply to Objection 2. The greater goods are not definitely fixed in the individual; but those which are simply and absolutely the greater good in general are fixed: and to these all the above particular goods may be reduced, as stated above.
Reply to Objection 3. Even the counsel of obedience is understood to have been given by Our Lord in the words: "And [let him] follow Me." For we follow Him not only by imitating His works, but also by obeying His commandments, according to John 10:27: "My sheep hear My voice... and they follow Me."
Reply to Objection 4. Those things which Our Lord prescribed about the true love of our enemies, and other similar sayings (Matthew 5; Luke 6), may be referred to the preparation of the mind, and then they are necessary for salvation; for instance, that man be prepared to do good to his enemies, and other similar actions, when there is need. Hence these things are placed among the precepts. But that anyone should actually and promptly behave thus towards an enemy when there is no special need, is to be referred to the particular counsels, as stated above. As to those matters which are set down in Matthew 10 and Luke 9 and 10, they were either disciplinary commands for that particular time, or concessions, as stated above (2, ad 3). Hence they are not set down among the counsels.
Source: Summa Theologica (New Advent)