Parashat Vayeshev describes Potifar's attempt to seduce Yosef and his heroic resistance to her efforts. The verse states, "He refused; he said to his master's wife... How could I do this great evil, and I will have sinned against God?'"
Generally, the verse is understood as if a colon appeared after the opening word of the verse - "Vayema'en" ("He refused"). That is, the rest of the verse describes his refusal, expressed in his comments to Potifar's wife. However, Rav Itzele' of Volozhin notes that a "pesik" (separation mark) appears after the word "Vayema'en." Additionally, the unusual cantellation note, the "shalshelet," which dictates the tune to be chanted with the word "Vayema'en," sets the word apart from the rest of the verse.
Rav Itzele therefore suggests that the verse in fact tells of two different stages of Yosef's refusal to Potifar's wife. Firstly, he refused, no questions asked. His instinctive response was an unquestionable, unequivocal "No!" This reaction involved no thought process whatsoever; the act itself was so unthinkable that not a millisecond of hesitation was ever entertained. Thereafter, Yosef proceeded to explain to Potifar's wife why he could not satisfy her wishes.
If this interpretation is correct, then Yosef's response to Potifar's wife involved, first and foremost, instinct. Yosef's impulses had been conditioned by his moral sense and commitment to Torah, to the point that inappropriate behavior could not even be entertained.
For us, perhaps the lesson is the constant reinforcement of the ideals of Torah and mitzvot. By studying Torah, we learn what God expects of us, we learn what He deems appropriate and what not. This process conditions our characters accordingly, as we incorporate within us the values of the Torah. The more we learn and develop a sense of right and wrong, the more these principles become part of our natural instinct, and the more we will be naturally inclined to pursue mitzvot and avoid aveirot.