Written in 1792 and not previously published, this essay sets out Reid's considered view of the nature of agency and the related idea of natural causation. Locke is wrong in supposing that the idea of power is acquired through external or internal experience, Hume in supposing that because it is not got through outer or inner sense we have no such idea. The primary conception of power is that of personal agency, got from the experienced fact that certain events are produced when we will to produce them. The concept of cause is related to this through the experience of the efficacy of volitional power. The problem, then, is what to make of the idea that nature contains non-volitional causes. An orderly world implies the operation of an intelligent agent. The idea of agency proper implies two-way operation, e.g., to be possessed of the power of walking is thereby to be possessed of the power of not walking. So a power does not necessitate a certain outcome, and what we think of as necessitating natural causes are related to natural regularities, rather than part of the idea of causality as such.