The Path of Purification
Suppose there were three people, a child without discretion, a villager, and a money changer, who saw a heap of coins lying on a money changer's counter. The child without discretion knows merely that the coins are figured and ornamented, long, square or round; he does not know that they are reckoned as valuable for human use and enjoyment. And the villager knows that they are figured and ornamented, etc., and that they are reckoned as valuable for human use and enjoyment; but he does not know such distinctions as 'This one is genuine, this is false, this is half value'. The money changer knows all those kinds, and he does so by looking at the coin, and by listening to the sound of it when struck, and by smelling its smell, tasting its taste, and weighing it in his hand, and he knows that it was made in a certain village or town or city or on a certain mountain or by a certain master. And this may be understood as an illustration.
Perception is like the child without discretion seeing the coin, because it apprehends the mere mode of appearance of the object as blue and so on. Consciousness is like the villager seeing the coin, because it apprehends, the mode of the object as blue, etc., and because it extends further, reaching the penetration of its characteristics. Understanding is like the money changer seeing the coin, because, after apprehending the mode of the object as blue, etc., and extending to the penetration of the characteristics, it extends still further, reaching the manifestation of the path.
That is why this act of understanding should be understood as 'knowing in a particular mode separate from the modes of perceiving and cognizing'. For that is what the words 'it is understanding in the sense of act of understanding' refer to.
However, it is not always to be found where perception and consciousness are. But when it is, it is not disconnected from those states. And because it cannot be taken as disconnected thus, 'This is perception, this is consciousness, this is understanding', its difference is consequently subtle and hard to see. Hence the venerable Nagasena said: 'A difficult thing, O King, has been done by the Blessed One. What, venerable Nagasena, is the difficult thing that has been done by the Blessed One? The difficult thing, O King, done by the Blessed One was the defining of the immaterial states of consciousness and its concomitants, which occur with a single object, and which he declared thus: "This is contact, this is feeling, this is perception, this is volition, this is consciousness" ' (Miln. 87).