Commentators over the centuries have explored the implications of the repeated word tzedek, or justice, in this verse, but as translator Robert Alter notes, "its function as a verbal gesture of sheer emphasis is self-evident: justice, and justice alone, shall you pursue." Let's consider two aspects of this verse that will aid our mussar practice.
First, justice is a standard, a knowable measure by which we may, no, by which we must order our affairs. When we take account of our souls, it is not on the basis of spiritual feelings or sentiment, no matter how lofty. Rather, it is on the basis of the moral standards that permeate Deuteronomy and the whole of Scripture. Deuteronomy makes it clear that the justice rooted in the worship of the one true and living God will be expressed in our treatment of our fellow human beings.
Yeshua tells us, "Unless your righteousness exceeds the righteousness of the scribes and Pharisees, you will by no means enter the kingdom of heaven" (Matt. 5:20). He goes on to show how righteousness, or justice—for tzedek can be translated as either—is defined in Scripture, and revealed in how we treat and think about and respond to those around us. Messiah calls and empowers us through his Spirit to fulfill the words of Scripture in heart and action. For our righteousness to exceed that of the scribes and Pharisees, what is written must become what is lived. Here is an aid to heshbon ha-nefesh. Take a daily inventory of your actions and thoughts toward those around you. Ask yourself, "How did I behave today toward my fellow human beings? How did I think and speak of my fellow human beings, created in the image of God just as I am?" Record your answers in a notebook or journal to help take inventory and mend your ways.
The second word we should consider is "pursue." Apparently, we never completely achieve justice. We live in a world that constantly overthrows justice, a world in which might often seems to prevail over right. This same world defeats us personally from time to time. We compromise our values or fail to act upon them. God in his mercy provides teshuvah—a way of return to him and his ways. Teshuvah is essential because in the world we live in, we must pursue justice continually.
Tzedek is never passive, as if we could pursue it by simply avoiding wrongdoing. In the Midrash on Deuteronomy 16:18, Rav Judah and Rabbi Nahman discuss the verse, "And David executed judgment [mishpat] and righteousness [tzedakah] unto all his people" (2 Sam. 8:15). One of them says, "David executed judgment, [in that] he acquitted the innocent and condemned the guilty; if, however, the guilty party had not the means to pay [the sum adjudged] he would pay it himself. This is the force of 'judgment and righteousness.'" We must actively pursue righteousness, not simply avoid unrighteousness.
Hence, Yeshua also instructs us,
Blessed are those who hunger and thirst for righteousness, For they shall be filled. (Matt. 5:6)
As we grow in our practice of mussar, may the Spirit of God give us a longing for righteousness and the power to pursue it, as Messiah teaches us to do.