The Sin of Adam - inaccurate self-assessment

Rav Tzadok (Machshovos Charutz #1):.. The sin of Adam was in thought rather than deed…These are the thoughts that the Serpent gave him when he told him, “You will be like G‑d.” The source of all vain and empty thoughts is that a person errs in self‑assessment. He mistakenly thinks that he is on a higher level than he is actually is or is even capable of reaching. Adam who had reached the pinnacle of perfection as a man, mistakenly started thinking that he was in fact on the level of G‑d. Therefore the Torah warns that while a person should be holy he can never reach G‑d's level of holiness (Vayikra Rabbah 24:9). When he had in fact reached the level of the angels, his evil inclination did not leave him alone and deceived him into thinking that he could be as holy as G‑d. A person can thus achieve the pinnacle of greatness and yet fall to the deepest depths – as we see what happened to Yeshu and Shabtsai Tzvi. Their great asceticism and separation from the material world distorted their perception of their true spiritual level until they thought that they were the source of holiness…" This is the way of the seduction of the Serpent (evil inclination). He begins with something small and progresses. He begins with thoughts and imaginations which seem external and superficial but in fact touch all levels of thought in an essential manner.