A being that is active or operative in the production of a change is said to be the efficient cause of the change, the latter being termed the effect. Now the greater the change, _i.e._ the higher and more perfect be the grade of reality that is actualized in the change, the higher too in the scale of being must be the efficient cause of that change. There must be a proportion in degree of perfection or reality between effect and cause. The former cannot exceed in actual perfection the active power, and therefore the actual being, of the latter. This is so because we conceive the effect as being produced or actualized _through the operative influence_ of the cause, and _with real dependence_ on this latter; and it is inconceivable that a cause should have power to actualize other being, distinct from itself, which would be of a higher grade of excellence than itself. The nature of efficient causality, of the influence by which the cause is related to its effect, is not easy to determine; it will be discussed at a subsequent stage of our investigations (ch. xi.); but whatever it be, a little reflection should convince us of the truth of the principle just stated: that an effect cannot be more perfect than its cause. The mediæval scholastics embodied this truth in the formula: _Nemo dat quod non habet_—a formula which we must not interpret in the more restricted and literal sense of the words _giving_ and _having_, lest we be met with the obvious objection that it is by no means necessary for a boy to have a black eye himself in order to give one to his neighbour! What the formula means is that an agent cannot give to, or produce in, any potential subject, receptive of its causal influence, an actuality which it does not itself possess virtually, or in its active power: that no actuality surpassing in excellence the actual perfection of the cause itself can be found thus virtually in the active power of the latter. There is no question of the cause or agent transferring bodily as it were a part of its own actuality to the subject which is undergoing change(75); nor will such crude imagination images help us to understand what real change, under the influence of efficient causality, involves.(76) An analysis of change will enable us to appreciate more fully the real difficulty of explaining it, and the futility of any attempt to account for it without admitting the real, objective validity of the notions of actual and potential being, of active powers or forces and passive potentialities in the things that are subject to change.