Thus the concept of life—i. e., of self-sustaining and self-generated action—is of vital (pun intended) importance to our understanding of reality, including in particular three classes of entities that are generated by life functions:

Organisms. Because an organism is sustained by its own action, that action is the function that explains its existence and nature. Living organisms have a functional organization, in which virtually every major part, organ, or behavioral pattern is determined and structured by the organism's requirements of self-maintenance and self-generation, often down to a remarkable level of detail. (The only major exception is reproductive systems, which ensure the continuance of the species as a whole.) Of course, living entities are not perfectly structured or free from defects. Nevertheless, any understanding of their structure and existence must begin with the fact that such an entity is adapted to the requirements of its own survival and growth. As noted in (3) below, however, species that have been bred by humans, such as dogs, may also exhibit traits that are better explained in terms of their function in human life.