Every living organism has a means of survival.
Plants survive by means of purely physical functions. They acquire the objects they seek, such as food, water, and sunlight, from the soil and air in which they grow, without the need of awareness. For every form of life above this level, however, consciousness is the basic means of survival.
The lower conscious species (e.g., jellyfish or flatworms) appear to have only the faculty of sensation and act by responding to isolated, momentary stimuli; their guide to sustaining their life is the pleasure-pain mechanism built into their bodies. The higher animals are also guided by the pleasure- pain mechanism, but in their case it functions within the context of the faculty of perception. The higher animals grasp and deal with the world of entities (and are able to form automatic perceptual associations). The range of actions required for their survival is therefore wider. They have to learn. a set of vital skills, such as hunting, storing food, hiding, or nest-building, which are impossible to the purely sensory species.
Man, too, experiences the sensations of pleasure and pain, but he is a …
Read the rest in Objectivism: The Philosophy of Ayn Rand.