Dietrich Bonhoeffer

At the end of Dietrich Bonhoeffer's Ethics, there is an analysis of the Lutheran symbolic literature (ie. the Book of Concord: the Augsburg Confession, Luther's Catechisms, etc.) Bonhoeffer, therein, discussed the Lutheran understand of the Law and asks whether there is one primary use of the Law (primus usus legis) or three?

I'll summarize Bonhoeffer's three uses of the law as:

1. The First Use (primus usus legis) is the revelation of God's Law of Truth to man for knowing his Divine Will
2. The Second Use (duplex usus legis) is judicial, and for the purpose of restraining the ungodly and sinner, and condemning those who do not live according to the First Use.
3. The Third Use (tertius usus legis) is for the regenerate man to put to death his sinful nature, and is used as a help for man in his sinful state living under grace, to know how to live rightly.

The "Third Use of the Law" is the sanctification of our Christian Life, and helps us put to death our sinful flesh, and free us from lawlessness. Bonhoeffer's question demonstrates that the Third Use is so primarily, that the question of whether it is really a Third Use at all, but the Primary One Use. Is the Third Use any different, or just another way of understanding all of the Law. 

Here's the quotation from Ethics:

"The law of God is "a divine doctrine in which the just and unalterable will of God is revealed as to how man is to be framed in his nature, his thoughts, his words and his works, so that it may be pleasing and acceptable to God; and transgressors are threatened with the wrath of God, with punishment in time and in eternity" (S.D. V,17) (duplex usus here?). This primus usus legis concerns the establishment of a disciplina externa et honestas (S.D. VI, 1). The secundus usus concerns the concerns the knowledge of sins. The tertius usus serves as a rule of conduct for converts and as punishment for the flesh, which is still alive even in them. Even though the symbolic writings do not explicitly exclude it, it is in the nature of the case not possible to substantiate the view that distinction between the three usus relates to the chronological succession in the proclamation or to two fundamentally different classes of men, believers and unbelievers, in that order. The externa disciplina still applies also to the believers; and so do the threat and punishment of the law, so far as the believer is still flesh ("for the old Adam, the obstinate and contentious ass, is still also a part of them which cannot be subdued and made obedient to Christ solely with the teaching of the law or by admonition, by driving and by threatening, but which often also requires the cudgel of punishments and torments" (S.D. VI, 24). The believer also still has need of the knowledge of sin through the law. Furthermore  already the primus usus comprises the whole contents of the law, namely, the entire decalogue. It also contains the threat and the promise which are addressed to the transgressor and the performer of the law respectively. It is uncertain whether or not the usus paedagogicus acquires an independent significance as a fourth usus between the primus and secundus usus; the Schmalkalden Articles recognize only two usus. These facts make it clear that the distinctions between the usus must not be understood chronologically; they do not refer to fundamentally different classes of men; they must be understood in relation to their substance. The primus usus defines the contents of the law with reference to the accomplishment of certain particular external works; the secundus usus defines the relation between the law and the person, leading the person to recognize that he is in opposition to the law and is condemned for it; the tertius usus defines the law as God's merciful help in the performance of the words which are commanded. The primus usus is the law as the preaching of works; the secondus is the law as the preaching of the knowledge of sin; the tertius is the law as preaching of the fulfilment of the law. The proclamation of the law always comprises all three elements; if it is otherwise necessary to ask not only whether it is expedient to employ the concept of usus (§1) bu also whether the doctrine of usus is justified theologically (vide infra §12).

- Dietrich Bonhoeffer, trans. Neville Horton Smith, Ethics, pgs 300-302

 Institutes 2.7.12. The third use of the Law (being also the principal use, and more closely connected with its proper end) has respect to believers in whose hearts the Spirit of God already flourishes and reigns. For although the Law is written and engraven on their hearts by the finger of God, that is, although they are so influenced and actuated by the Spirit, that they desire to obey God, there are two ways in which they still profit in the Law. For it is the best instrument for enabling them daily to learn with greater truth and certainty what that will of the Lord is which they aspire to follow, and to confirm them in this knowledge; just as a servant who desires with all his soul to approve himself to his master, must still observe, and be careful to ascertain his master’s dispositions, that he may comport himself in accommodation to them. Let none of us deem ourselves exempt from this necessity, for none have as yet attained to such a degree of wisdom, as that they may not, by the daily instruction of the Law, advance to a purer knowledge of the Divine will. Then, because we need not doctrine merely, but exhortation also, the servant of God will derive this further advantage from the Law: by frequently meditating upon it, he will be excited to obedience, and confirmed in it, and so drawn away from the slippery paths of sin. In this way must the saints press onward, since, however great the alacrity with which, under the Spirit, they hasten toward righteousness, they are retarded by the sluggishness of the flesh, and make less progress than they ought. The Law acts like a whip to the flesh, urging it on as men do a lazy sluggish ass. Even in the case of a spiritual man, inasmuch as he is still burdened with the weight of the flesh, the Law is a constant stimulus, pricking him forward when he would indulge in sloth. David had this use in view when he pronounced this high eulogium on the Law, “The law of the Lord is perfect, converting the soul: the testimony of the Lord is sure, making wise the simple. The statutes of the Lord are right, rejoicing the heart: the commandment of the Lord is pure, enlightening the eyes,” (Ps. 19:7, 8). Again, “Thy word is a lamp unto my feet, and a light unto my path,” (Ps. 119:105). The whole psalm abounds in passages to the same effect. Such passages are not inconsistent with those of Paul, which show not the utility of the law to the regenerate, but what it is able of itself to bestow. The object of the Psalmist is to celebrate the advantages which the Lord, by means of his law, bestows on those whom he inwardly inspires with a love of obedience. And he adverts not to the mere precepts, but also to the promise annexed to them, which alone makes that sweet which in itself is bitter. For what is less attractive than the law, when, by its demands and threatening, it overawes the soul, and fills it with terror? David specially shows that in the law he saw the Mediator, without whom it gives no pleasure or delight.