Musar על פסחים 100:13

But still, one should not despair, even one for whom it is difficult to cleanse his thoughts and to entirely remove lewd thoughts from his heart. For if so, even His mitzvoth and his Torah would not avail to sanctify his soul, and how would he ever attain his end? But the major element here is Chazal's formula: "If one comes to cleanse himself he is helped [by Heaven]." And if he desires with his whole heart to remove lewd thoughts from it, he will certainly be aided by Heaven to do this, and he must not, G-d forbid, nullify any mitzvah or Torah, even though his thoughts are not entirely pure. In this connection Chazal have said (Pesachim 50b): "Let one always occupy himself with Torah and mitzvoth, even lo lishmah [not for the sake of Heaven], for from lo lishmah, lishmah [for the sake of Heaven] will follow." That is, the holiness of the Torah and the mitzvah will help him, so that it will be in his power to do and to learn for the sake of Heaven, too. And this is what is alluded to in the above verse itself. For, in the beginning it is written (Bamidbar 15:39): "And you will remember all the mitzvoth of the L-rd and you will do them." "the mitzvoth of the L-rd," implying for the sake of the L-rd, is written only in respect to remembering, but not in respect to doing, his not yet having attained purity of thought in the doing of the mitzvoth. In any event, the doing of these acts will bring him to remove the [impure] thoughts of the heart and to overcome his lusts, after which he will reach the level of (Ibid. 40): "and you will do all of My mitzvoth," the doing, too, being exclusively for the sake of the L-rd; and this will bring him to the level of holiness— "and you will be holy." However, all of this obtains only with one who comes to purify himself and strengthens himself to remove the lewd thoughts and to keep himself from [the gratification of] his lusts. (Only) then does "He who comes to purify himself" obtain.]

The quality of shame is a fence and an iron partition between a man and all manner of sins, for a man may commit many sins in private that he would be ashamed to do in public. An example cited by our Sages: "When Rabbi Johanan ben Zakkai was about to expire, his disciples said to him, 'Our teacher, bless us!' And he said to them, 'May the fear of Heaven be upon you as much as you fear human beings!' They asked in wonder, 'Shall we indeed fear God only as much as we fear human beings and no more?' Whereupon, he said to them, 'Would that it were so, If only you would realize that when a man commits a sin he thinks "I hope no one sees me," (Berachoth 28b). All because he is afraid of being disgraced in the eyes of people, and this fear of disgrace often restrains him from doing a sinful act. Eventually, he will refrain from doing a sinful act from nobler motives (Pesahim 50b).

The second is a person who is ashamed to do wrong publicly and holds back from sinning even privately because he feels that somehow his sin will become known and then he will be ashamed before God, Blessed is He, and man (for his fault will be a profanation of the Name.) Of this person our Sages said: "From a base motive he will arrive at following a precept for its own sake" (Pesahim 50b).

According to the Midrash of Rabbi Nechunyah referred to earlier, Song of Songs describes the idyllic state of affairs when Israel studies both the written and the oral Torah for their own sakes, i.e. לשמה. This results in a union between the forces on earth and their counterparts in the Celestial Regions, and will release all of G–d's blessings onto the Jewish people.

There is another type of "not for the sake of the mitzva itself" (Shelo Lishma), which is the "for the sake of receiving a reward" (Avot 1:3). On this our sages said: "a man should always occupy himself with Torah and good deeds, even if it is not for their own sake, for doing so will lead to doing them for their own sake" (Pesachim 50b). Nevertheless, he who has not yet reached from "not for their own sake" (shelo lishma) to "for their own sake" (lishma), is still far from reaching his Shelemut (wholeness/perfection).

Concerning this matter our Sages said : "There is an alert person who receives a reward and there is an alert person who loses by his alertness. There is a lazy man who receives a reward, and there is a lazy one who loses by his laziness. The alert person who receives a reward is one who does his work all week long but does not do any work on the eve of the Sabbath. The alert person who loses by his alertness is one who does work all week long, and also works on the eve of the Sabbath. A lazy person who receives a reward is one who loes not do work all week long and does not do work on the eve of the Sabbath either. A lazy person who loses thereby is one who does not do any work all week, but does work on the eve of the Sabbath" (Pesahim 50b). Observe how the sages came up with the idea that there is an alertness that is good and an alertness that is bad. It is the same with laziness.

And thus did our Rabbis interpret the text, "The poor man's wisdom is despised" (Eccl. 9:16). What wisdom of what poor man is despised? It refers to a man who is poor in good deeds; then his wisdom is despised. And if one teaches laws that instruct us to do good and he himself does not do good deeds his words are not accepted, for "not learning but doing is the chief thing" (Aboth 1:17). Wherefore it is necessary tor every man to direct his deeds for the sake of Heaven and see and understand from the example of Elisha, the son of Abuyah : because his father taught him Torah not for its own sake, the son turned to heresy. As it is said in the Jerusalem Talmud (Hagigah 2:1), and in the Midrash (Eccl. Rabbah 7:8, letter 18) : When Elisha, the son of Abuyah was circumcised his father made a feast for the Sages and invited Rabbi Eiezer and Rabbi Joshua and the other Sages, and they sat at the feast and spoke words of the Torah to the extent that a fire came and encircled them. The father of Elisha came and said to them, "My good friends, did you come to burn down the house?" And they said to him, "The fire that you see is only because we are studying the words of the Torah and the Prophets and the Scriptures. And the words of our discussion joyously testify to the truth just as when they were given at Mount Sinai. From Mount Sinai were they given, and from the fire were they given!" The father of Elisha ben Abuyah then said, "Now that I see the power of the Torah, if this my son will live on, I shall give him to the study of Torah." And because the father had it in his mind that he would teach his son Torah not for its own sake but for the power it would give him, his son became a heretic. And even so, a man should study the Torah even if not for its own sake, because from studying it not for its own sake, he will ultimately come to study it for its own sake (Pesahim 50b).

We have a principle (Pesachim 50b) that studying Torah for personal rather than pure motives will eventually lead a person to study Torah for the proper motives. We also have a statement (Deut. 10,17) that G–d does not respond to attempts at bribery. If both statements are taken at face value one could gain the impression that the person studying Torah for ulterior motives will be rewarded by G–d by eventually studying Torah for pure motives. This would be similar to G–d having been bribed by the person studying for personal motives. The author therefore understands the statement מתוך שלא לשמה בא לשמה to mean that during the course of Torah study one becomes aware that G–d detests people who use Torah study for their own personal advantage. Once a student realizes this he will only study Torah for pure motives in the future. Torah study for ulterior motives per se is a sin, however. These and other quotations from the Torah indicate how much perfection of one's נפש is expected of us.