[This is the 3rd in a series of posts summarizing Ayn Rand's 1961 article, "The Objectivist Ethics". The first post is here.]
A human first becomes aware of the issue of "good or evil" through the sensations of pleasure and pain, which are experienced automatically according to the standard of his or her life. Pleasure signifies a correct course of action; pain is a warning to do something different. This is one form in which consciousness is a conscious organism's basic means of survival.
Plants automatically pursue their goals through biochemical mechanisms, drawing in their nourishment and processing it by built-in physical means.
The lowest forms of conscious organisms (such as worms) use sensations to automatically direct their actions and pursue their goals. A sensation is an automatic, fleeting product of a sense organ's reaction to a stimulus.
Animals are able to retain sensations in the form of percepts, enabling them to have much more complex behaviors than the lowest conscious species. A percept is a group of sensations automatically retained and integrated by the brain of a living organism. (ITOE 1.4.1) Perceptions constitute an animal's awareness of entities, an awareness it cannot avoid and for which it has no choice of how to react: an animal's instincts program it to pursue its own survival.