The understanding of variability in behavior is extraordinarily difficult because behavior consists of actions that are purposeful processes directed to reach future results and psychological functions can deliberately be adjusted for this. The conventional method used in handling this problem is to make behavior in experiments as similar as possible to processes studied in the natural sciences. It is suggested this allows the revealing of simple mechanisms of behavior that are independent of purpose and deliberation. A sufficient basis of the simple mechanisms should elucidate purposefulness mechanistically. It is implicitly assumed this method defines the simple mechanisms unequivocally. The replication crisis hints this assumption is incorrect. Arocha (2021) suggests purpose is the essential component in understanding behavior. However, Arocha assumes no mechanistic explanations for goal-directed processes, thus restricting the usefulness of his ideas. I suggest the goal and means of an action are constructed jointly through the criterion of minimal construction costs. This mechanistically determines actions.