Purpose is deliberately thought-through goal-directedness.
According to some philosophies, purpose is central to a good human life. Helen Keller wrote that happiness comes from "fidelity to a worthy purpose", and Ayn Rand wrote that purpose must be one of the three ruling values of human life (the others are reason and self-esteem). Some people think that God assigns purposes to people and that it is their mission to fulfill them. Others say that purpose is not inherent, but instead freely chosen (or not chosen) by individuals. Among these, some say that natural propensities may determine what sorts of purposes a person needs to pursue, but do not guarantee that he or she will pursue them, that being dependent on free choice.
Pursuing a career, raising a family, devotion to a creative vocation, and acquiring property are perhaps the most widespread of long-term purposes that make life meaningful according to such philosophies. Public service and helping the needy are often cited. Variants of philosophies such as eudaimonia and objectivism sometimes claim that self-sacrificial goals are destructive.
Purpose is similar to teleology, the idea that a final goal is implicit in all living organisms. Until the modern age, philosophy followed Aristotle's depiction of a teleological cosmos in which all things had a final purpose, (namely, to realise their implicit perfection). Perhaps most modern philosophers of science have reversed the idea of purpose inherent in nature; they do not consider an eye explicable as being "in order to see"; instead, cause-and-effect processes are credited with bringing about the eye organ, which allows us to see. The difference is between a cause as pushing from behind (movements of billiard balls) and a cause as pulling from within (movement of a growing plant). With teleology (purpose) matter is fulfilling some aim from within.
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