Never Ceasing Pursuit Of Truth
צדק צדק תרדוף
"Tzedek, tzedek tirdof" — "Justice, justice shall you pursue" (Devarim 16:20).
The pasuk's doubling of the word "justice" hints that pursuing tzedek is not a one-time act but a continuous demand.
פי' אחר שזוכין להשיג מעט אמת צריכין לשמור שלא להניח מלרדוף אחר האמת
The explanation is that once a person merits to grasp a small measure of emes, he must take care never to cease pursuing the truth further.
Once a person has acquired even a little bit of truth, he is now responsible to keep striving after more and never to grow complacent.
כי אם ח"ו יורדין לשקר אח"כ
For if, chas v'shalom, he afterward descends into falsehood,
The danger is that a person who stops pursuing the truth can slide backward into falsehood.
מוטב הי' קודם שהשיג זה האמת
it would have been better had he never attained this measure of emes to begin with.
Such a fall is so severe that it would have been preferable never to have reached that level of emes in the first place, since now there is something to lose.
ולכן הצדק שמבקשין אחר מציאת צדק הראשון הוא רדיפת הצדק באמת
Therefore, the very righteousness that one seeks after having found the first measure of justice is itself the genuine, ongoing pursuit of justice in emes.
The repeated "tzedek" teaches that true righteousness lies precisely in the ongoing chase after truth even after one has already attained some.
כמ"ש בתורה לשון זה כמה פעמים שמוע תשמעו שמור תשמרון
This is as the Torah employs this doubled form of expression many times — "shamo'a tishme'u," "if you will surely hearken," and "shamor tishmerun," "you shall surely guard."
The Sfas Emes notes that the Torah often uses this doubled verb form to convey a continuous, unceasing obligation.
וכמ"ש לעיל כמה פעמים מזה כי הכפל הוא העיקר כמאמר שנה הכתוב לעכב:
And as was explained above several times regarding this matter, that the doubling is the essential point, in keeping with the dictum of Chazal that when the verse repeats itself it does so to make the matter binding and indispensable.
He references his earlier teachings that the doubling itself carries the main lesson, based on Chazal's principle that a repeated verse comes to make the matter firm and obligatory.
Summary: On the pasuk "Tzedek, tzedek tirdof," the Sfas Emes explains that the doubling of "justice" teaches that the pursuit of emes must never stop. Once a person merits to grasp even a small portion of truth, he bears a heightened responsibility to keep pursuing it, for if he ceases he is liable, chas v'shalom, to slide into falsehood — a fall so grave that it would have been better never to have reached that level at all. The genuine avodah, then, is the ongoing, unending chase after truth that continues even after one has already attained some. The Sfas Emes connects this to the Torah's frequent use of doubled verbs such as "shamo'a tishme'u" and "shamor tishmerun," noting per Chazal that the repetition is itself the essential point, coming to make the matter binding and indispensable.