§ 43. Skill may be said to be _free_ when the essential terms of the action have been abstracted from the circumstances attending them in individual experiences, and are retained as ideal plans applicable to any practical occasion. The monkey who swings with a trapeze from his perch on the side of the cage, counts upon swinging back again without any further effort on his own part. His act and its successful issue signify his practical familiarity with the natural motions of bodies. We can conceive such a performance to be accompanied by an almost entire failure to grasp its essentials. It would then be necessary for nearly the whole situation to be repeated in order to induce in the monkey the same action and expectation. He would require a similar form, color, and distance. But he might, on the other hand, regard as practically identical all suspended and freely swinging bodies capable of affording him support, and quite independently of their shape, size, time, or place. In this latter case his skill would be applicable to the widest possible number of cases that could present themselves. Having a discerning eye for essentials, he would lose no chance of a swing through looking for more than the bare necessities. When the physicist describes the pendulum in terms of a formula such as t = 2pi[squareroot(l/g)] he exhibits a similar discernment. He has found that the time occupied by an oscillation of any pendulum may be calculated exclusively in terms of its length and the acceleration due to gravity. The monkey's higher proficiency and the formula alike represent a knowledge that is free in the sense that it is contained in terms that require no single fixed context in immediacy. The knowledge is valid wherever these essential terms are present; and calculations may be based upon these essential terms, while attendant circumstances vary _ad infinitum_. Such knowledge is said to be _general_ or _universal_.