The Cloud of Unknowing, ed. by Evelyn Underhill, , at sacred-texts.com
Of the powers of a soul in general, and how Memory in special is a principal power, comprehending in it all the other powers and all those things in the which they work.
MEMORY is such a power in itself, that properly to speak and in manner, it worketh not itself. But Reason and Will, they be two working powers, and so is Imagination and Sensuality also. And all these four powers and their works, Memory containeth and comprehendeth in itself. And otherwise it is not said that the Memory worketh, unless such a comprehension be a work.
And therefore it is that I call the powers of a soul, some principal, and p. 276 some secondary. Not because a soul is divisible, for that may not be: but because all those things in the which they work be divisible, and some principal, as be all ghostly things, and some secondary, as be all bodily things. The two principal working powers, Reason and Will, work purely in themselves in all ghostly things, without help of the other two secondary powers. Imagination and Sensuality work beastly in all bodily things, whether they be present or absent, in the body and with the bodily wits. But by them, without help of Reason and of Will, may a soul never come to for to know the virtue and the conditions of bodily creatures, nor the cause of their beings and their makings.
And for this cause is Reason and Will called principal powers, for they work in pure spirit without any manner of bodilyness: and Imagination and Sensuality secondary, for they work in the body with bodily instruments, p. 277 the which be our five wits. Memory is called a principal power, for it containeth in it ghostly not only all the other powers, but thereto all those things in the which they work. See by the proof. p. 278