UNDERSTANDING, is defined by the Peripatetics, to be a faculty of the reasonable soul, conversant about intelligible things, considered as intelligible. They also make it twofold, viz. active and passive. Active understanding, they hold that faculty of the soul by which the species and images of intelligible things are framed, on occasion of the presence of phantoms or appearances thereof. For maintaining the intellect to be material, they hold it impossible it should be disposed to think by any disproportionate phantoms of mere body, and therefore that it is obliged to frame other proportionate species of itself, and hence its denomination active. Passive understanding, is that which, receiving the species framed by the active understanding, breaks forth into actual knowledge.
The moderns set aside the Peripatetic notion of an active understanding. The Cartesians define the understanding to be that faculty whereby the mind conversing with, and as it were intent on, itself, evidently knows what is true in any thing not exceeding its capacity. The Corpuscular Philosophers define the understanding to be a faculty expressive of things which strike on the external senses, either by their images or their effects, and so enter the mind. Their great doctrine is, Nihil esse in intellectu quod non prius fuerit in sensu; and to this doctrine Mr. Locke, and most of the latest English philosophers, subscribe.
Between the Cartesians and Corpuscularians there is this farther difference, that the latter make the judgment to belong to the understanding, but the former to the will. Hence, according to the most approved opinions of the Corpuscularians, the understanding has two offices, viz. perception and judgment; according to the Cartesians, it has only one, viz. perception. See METAPHYSICS AND LOGIC.