Possibility, too, may pertain to the agent, as the capacity to act, or to the recipient of action, as the capacity to receive it. Their need for a determining factor is not equally evident.

The need for an external determining factor is well known in the case of the recipient’s capacity, since it is perceived through the senses in manufactured things and in many natural things. Yet doubt may arise concerning natural things, since most have the principle of their change within themselves. Thus, in many such cases, it is thought that the mover is the thing moved—the very thing moved—and that it is not self-evident that everything moved has a mover and that nothing moves itself. All this requires explanation, which is why the ancients investigated it.

As for the capacity residing in the agent, it may be thought, in many instances, to require no external determining factor to pass into actuality. For the agent’s transition from not acting to acting may often be thought not to constitute a change requiring something to bring it about—for example, a geometer’s transition from not practising geometry to practising it, or a teacher’s transition from not teaching to teaching.”