Adam and Eve ate from the tree of the knowledge of good and evil. This is a metaphor for when humanity evolved from a purely animal / instinctive mind to a human / thinking mind regarding actions. Before this point in time, there was only what occurred, not any good or evil attached to it. After this point in time, action could be labelled as having good or evil attached to it.
This wording is specifically chosen: good and evil are not absolutes; they are value judgements that we attach to actions. For example, a particular aboriginal society believes that a woman's father should deflower the woman the day before her "wedding" to prepare her for her new husband. By our standards, this is wrong, but not be any objective standards. These judgements can certainly come from a purely logical standpoint of what is best for society tempered with what is best for the individual. They can also come from thinking that may not be logical; for example, I support the arts so that the amount of beauty in the world might be increased. It is not really a logical decision, more of a feeling decision.
The intuitive mind can also be used in deciding what actions are good and evil. One must be careful to distinguish between what feels "good" and what feels "right", however.