A person does not sin unless he is seized by a spirit of folly (Sotah 3a).
Some people try to defend a misdeed by claiming "temporary insanity." The Talmud is telling us that while all wrongdoing does result from temporary insanity, people are still held accountable for their behavior.
No sane person would do things that are self-destructive. Small children who do not know any better may eat things that are harmful, but when adults submit to temptation and eat things that are harmful, they have essentially taken leave of their adult senses. This form of temporary insanity accompanies every wrong act.
Civil law does not accept ignorance as a defense, and although Jewish law does consider ignorance a mitigating factor, it holds a person responsible for being derelict in not having obtained the requisite knowledge and information necessary to act properly.
Jewish law holds that while true psychosis may be an exonerating factor, a non-psychotic person is capable of overcoming the "temporary insanity" that leads to wrongdoing. The Talmud states that in evaluating any act, we should calculate the gain from the act versus the loss it entails. A reasonable person will conclude that a brief pleasure of indulgence is certainly not worth the price, whether it is in terms of negative physical effects or of spiritual deterioration. People are certainly accountable for failure to exercise their reason and come to correct conclusions.