What was that external force that tempted Adam and led him to commit the sin of eating from the Tree of Knowledge?
Rav Chayim in Volozhin, in his Nefesh Ha-chayim (1:6), discusses the transition that took place within the nature of man after Adam's sin in the Garden of Eden. The tree from which Adam and Chava partook is called the etz ha-da'at tov va-ra – "the tree of knowledge of good and evil" (2:9,17). This is commonly understood to mean that partaking of this tree instills within a person an evil inclination, a proclivity towards sinful conduct. Whereas before the sin Adam was pure, free of lustful desires for sinful pleasures, after partaking from the forbidden tree he found himself drawn after physical gratification. However, this explanation gives rise to the obvious question of how Adam was driven to partake of the forbidden fruit in the first place. If man's evil tendency entered his being only after the sin, then how did the sin occur to begin with?
Rav Chayim claimed that undoubtedly, man possessed free will to choose between good and evil even before the sin. However, Rav Chayim writes, his ability to do evil was not part of his essence; it was not an instinctive, natural tendency as it became after his sin. He compares Adam's free will before the sin to the free will each of us possesses to cast oneself into fire. Of course, each person has the physical capability to do such a thing. Clearly, however, there is no internal suicidal instinct. Whatever might compel a person to jump into fire stems from an external force, rather than from an instinctive impulse. Thus, although Adam certainly had the freedom to choose between good and evil even before the sin, the lure of evil was external to his being, as opposed to an internal force within him, as it is now.
Still, the question remains, what was that external force that tempted Adam and led him to commit the sin?
Rav Eliyahu Dessler, in his Mikhtav Mei-eliyahu (vol. 2, Parashat Bereishit), suggests that at least on one level, Adam was lured by the temptation of challenge. He felt he could achieve far more if he would experience the inner drive to sin and then resist it. Adam thought that religious life without an instinctive, sinful tendency is too easy, too simple, and does not reflect a deeply-ingrained sense of subservience to God. He figured he could reach greater achievements by experiencing the sinful instinct and then working to suppress it. Adam's tragic mistake was underestimating the force of the yetzer ha-ra to which he willfully exposed himself, or overestimating his strength to resist it.
More generally, Rav Dessler adds, Adam succumbed to the yetzer ha-ra of curiosity. Man is by nature enchanted by the unknown and the mysterious. The "external force" of which Rav Chayim of Volozhin spoke perhaps refers to this enchantment with uncharted waters, the natural desire to behold or experience that which lies beyond one's familiar world. Even if Adam felt no physical drive to partake of the forbidden fruit, he was likely drawn after his curiosities, the drive for "adventures" and new experiences. This yetzer ha-ra existed within him even before his sin, and is responsible for the fall that occurred as a result of that sin. As punishment for succumbing to his curiosities, Adam – and all mankind – was destined to live in a constant state of exposure to the instincts Adam wished to experience, and we must forever struggle with those internal drives and restrain them to obey God's law.