I
"Hashem made man straight (yashar), but they sought many calculations" (Koheles 7:29). Originally, man was a straightforward creature, who was intellectually inclined to avoid sin, irrespective of the consequences. Later, mankind pursued sweet pleasures and avoided unpleasant conditions. This resulted in many calculations and schemes which entice to sin (Seforno).
Harav Elchanan Wasserman (Kovetz Ma'amarim, Emuna) explains that faith in Hashem the Creator is obvious. Just as a garment bears witness to the weaver, a door to the carpenter and a house to the builder, so the world bears witness to Hashem Who created it (Medrash cited in Bais Hamedrash I, 114). "The Heavens declare the glory of Hashem" (Tehillim 19:2). Why, then, are there so many atheists, including brilliant philosophers?
The answer is in the Torah: "Bribery blinds the eyes of the wise" (Devarim 17:19). A person's desires for pleasure blind him. One strays after his heart into heresy if he strays after his eyes after a desire for immorality (Brachos 12b). "Israel served idols in order to allow flagrant immorality" (Sanhedrin 63b). These heretical calculations of a person blinded by desire derail him from the straight path of his original state.
II
The aforementioned passuk in Koheles speaks of one man (Adam), and concludes that they (plural) strayed. "They" includes Chava, and together they planned to sin (Rashi).
The serpent said to Chava "You will not surely die" (Bereishis 3:4). The serpent pushed her until she touched the tree. He said to her "Just as there is no death by touching it, so there is no death by eating it" (Rashi). Chava accepted the serpent's argument and denied Hashem's statement "On the day you eat from it you will surely die" (2:17). Her sin was one of a lack of faith. However, the Torah later attributes Chava's sin not to lack of faith, but to desire. "Chava saw that the tree was good for eating and desirable in her eyes" (3:6). Harav Y.Y. Kanievsky (a.k.a. the Steipler, in Bircas Peretz 3:4) resolves this apparent contradiction by explaining that if not for her desire, Chava would have realized that Hashem's command not to eat from the tree did not prohibit touching it. Her lack of faith was caused by her desires.
III
"Yeshurun became fat and kicked" (Devarim 32:15). Yeshurun, a name of Am Yisrael, derives from Yashar, straight. Some say that its root is "Ashurenu", I will see (Bamidbar 24:17) (Ibn Ezra). The Seforno elaborates: "Even those who engage in analysis (iyun), called Yeshurun, as in 'I will see', acted like animals who kick people who feed them. You, Yeshurun, those who engage in Torah and analysis, turned to physical pleasures, and became fat ("shamanta"). You then became thick ("avisa") and unable to understand the fine points (dakus) of the truth. You (i.e. your eyes) became covered ("kasisa"), as it says in Yeshaya 44:18, "Their eyes are smeared (prevented) from seeing, their hearts from comprehending."
The intelligentsia of Am Yisrael was blinded by the desire for physical pleasure. They could no longer see or understand properly. As a result, the masses abandoned Hashem their Creator (Seforno).
This is a recurring theme read this week in Koheles, next week in Bereishis, and last week in Ha'azinu. One's desires are a form of bribery that blind and lead to heretical miscalculations, rationalizations in violating Hashem's command, and abandonment of Hashem. This affects all of mankind and all members of Am Yisrael, regardless of intellectual level.
Perhaps the two interpretations of Yeshurun are related. In order to see, a person must be straight, as he was created, and not overly involved in blinding physical pleasures. By avoiding the miscalculations caused by temptation, we can, and must, be straight and see straight.