299. Sense Knowledge. -- The different forms of sensation are studied as in Aristotle But to the internal senses, the aestimativa in the animal, or cogitativa in man, was now added. This is the sense which, instinctively in the former case and under the direction of the intellect in the latter, perceives the concrete relation of the useful or the hurtful. Vaguely hinted at by Aristotle, this faculty was treated to long commentaries by the Arabian philosophers; and it is from the latter that the teaching of the thirteenth-century scholastics draws its inspiration.

The seat of sensation is the organism, that is to say, the body "informed" by the soul. Under the influence of two distinct currents of Arabian thought, issuing the one from Monte Cassino (126), the other from the Arabian schools of Spain (216), Western philosophers were inclined to fix their attention exclusively on the physiological side of sensation: more than one of them drifted dangerously near materialism in their conclusions. But the scholastics of the thirteenth century soon set things to rights: not ignoring the physiological aspect of the process, they took care to give the psychical aspect its true value as well; they proclaimed the irreducibility of the two phenomena, the physiological and the psychical; while at the same time they recognized in the interdependence of these phenomena a fundamental law of sense life, and, consequently, of all perceptive and appetitive activity.

While the Augustinians considered sensation as a psychical phenomenon which the soul produces in itself on the occasion of a sense impression, Albert the Great, St. Thomas Aquinas, St. Bonaventure even, Henry of Ghent, Duns Scotus -- in a word, all the great scholastics -- adopted the peripatetic explanation of the genesis of sensation. This, briefly, is their theory: The sense faculties are passive powers.^{1} The stimulus of the exterior object gives them the initial impulse, without which they would remain inactive. On receiving this determining impulse, the passive faculty reacts, and this reaction completes the process of cognition, impressed and expressed (SPECIES SENSIBILIS IMPRESSA, EXPRESSA); or again, representation impressed from without, and expressed from within, are the terms employed to mark the two stages of the perceptive process. This latter is accomplished entirely within us, but parallel to this PSYCHIC doctrine, scholasticism also developed Aristotle's theory of the PHYSICAL MEDIUM (53).

The leading scholastics, St. Thomas and Duns Scotus, to mention no others, set forth in all their fulness and purity the genetic laws of sensation; they draw a clear distinction between the PSYCHIC transformation ("immutatio psychica") wrought by the object on the sense, and the PHYSICAL phenomenon which take place in the medium. But many of their predecessors and contemporaries were led astray by the false interpretation placed upon Aristotle's "species sensibilis" by certain of his commentators. For these, the "impressed species" is not a psychic determinant, an effect produced by the object and received in the faculty, but rather a miniature portrait of the external thing, a tiny image which traverses the intermediate space between the object and the organ, a substitute for the reality, coming finally into contact with the sense organ, and assimilated (intussusceptio) by the latter, thus producing knowledge. On disappearing from the field of consciousness, the sensation leaves a trace, an image (phantasma). This latter survives in the imagination, and its principal function is to contribute, in the absence of the object, to the production of thought.

{1} A technical expression, often misunderstood and misused. FROSCHAMMER, one of the historians of St. Thomas, failing to understand it aright, has accused the Angelic Doctor of making knowledge a purely passive phenomenon. Same error in ERDMANN, Gesch. Phil., i., p. 452 (Berlin, 1892); in WERNER, Joannes Duns Scotus (Vienna, 1881), p. 76. A passive faculty is not a non-acting faculty, but a faculty which is passive before being operative, a faculty which must be "informed" by something other than itself before exercising an activity: as opposed to the active power, which has no need of any such foreign influence, but passes to the act as soon as the conditions required for this purpose are verified.