In the Aristotelian terminology, the 'first actuality' is the being in readiness to act, the 'second actuality' is the being in action. A locomotive with steam up is in the 'first actuality ': a locomotive on its way is in the 'second actuality.' We must secure the 'first actuality' of science and skill, before we can exercise the 'second actuality ' of a skilful scientific investigation. St Thomas quotes Aristotle, De anima, II, i: "This actuality is understood in two senses: the first is represented by habitual knowledge, the second by the actual use of the understanding to mark a truth. Wherefore soul is defined, 'the first actuality of living body.'"

Of God and His Creatures: 2.60