'Every agent acts inasmuch as it is in actuality,' is a favourite axiom of the schoolmen, to which there is a reciprocal: 'Everything that is acted upon is acted upon inasmuch as it is in potentiality': which is also put thus, 'Everything received is received according to the mode of the recipient.' 'To be in actuality,' is something akin to the modern conception of 'energy.' Every agent then acts according to its proximately available energy. A man does only what he is 'up to doing.'

Of God and His Creatures: 1.16