Because the definition of behavior changes as our understanding of behavior changes, giving a final definition is impossible. One can, however, rule out some possibilities and propose some others based on what we currently know. Behavior is not simply movement, but must be defined by its function. Also, our understanding of behavior must agree with evolutionary theory. I suggest 4 basic principles: (a) only whole organisms behave; (b) behavior is purposive; (c) behavior takes time; and (d) behavior is choice. Saying that parts of an organism behave is nonsense, and, moreover, evolutionary theory explains the existence of organisms mainly through their adaptive behavior. Behavior is purposive in that behavior is shaped by its consequences, through an organism's lifetime or through interactions with the environment across many generations of natural selection. Behavior takes time in that behavior is interaction with the environment that cannot take place at a moment. Moreover, at a moment in time, one cannot definitely identify the function of behavior. Identification of an activity requires a span of time. Behavior is choice in the sense that a suitable span of time always includes time spent in more than 1 activity. Activities include parts that are themselves activities on a smaller time scale and compete for time. Thus, behavior constitutes time allocation. An accounting problem arises whenever any behavior is attributed to multiple consequences. In the molar multiscale view, this raises the question of whether 2 activities can occur at the same time. The question remains open.
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