Each of the parts of an organism is likewise generated and maintained by that organism in order to fulfill a need (or needs) of the organism. Consequently, we may say that the organ, cell, or other component is more or less "well-functioning"—"good" or "bad" in popular speech—depending on whether or not it satisfies that need properly. When a doctor tells her patient that he has a "bad heart," she is speaking about a scientifically determinable fact: that the heart is not pumping blood efficiently or may soon cease to do so, thus no longer serving a vital need of the patient. The standard of judgment she applies is not chosen arbitrarily, but derives from the natural function that explains why human beings and other advanced organisms have hearts—which is another fact known from scientific observation.

Using an identical approach, we can extend this concept to a whole organism. The activity of a living organism, as we have seen, arises from its necessity in satisfying the needs of the organism itself. That activity can therefore be described as well-functioning insofar as it successfully provides for those needs. Alternatively, we say that the organism itself functions "well" or "properly." A number of other evaluative terms are also commonly applied, based on the same criterion. If the organism does not act effectively to sustain its own life and health, it is less well-functioning—or, one may say, malfunctioning.