Origin Of Sensations: Psychical and Physical Aspects

The Philosophical System of Thomas Aquinas, by Maurice de Wulf, Edited & adapted for the Web by Dr. Jonathan Dolhenty

III. How Our Knowledge Is Formed. A. Origin of sensations – Psychical and physical aspects

There are still two important questions concerning the different kinds of knowledge which consciousness reveals to us: how they are formed and what is their value. These two questions are quite distinct, and form the subject of the following parts. Here we shall discuss how knowledge, whether sensuous or intellectual, comes into existence.

As soon as a child awakens to life, his external senses bring him into contact with something other than his consciousness: the color, taste, shape, resistance, temperature, etc., of material things. Throughout life, sensations continue to play this principal role. Now, according to the Schoolmen, a sensation necessitates an influx of a particular object known and the reaction of the subject knowing. Let us take the sight of an oak tree as an example. The sense or psychic power of sight does not derive from itself the content of its act of vision. An impulse coming from outside and received by me is an indispensable factor, without which an act of sight would be impossible. But as soon as that impulse coming from the oak tree is received in me, I react to the stimulus, and this vital reaction completes the sense perception. The whole phenomenon is imprinted from outside, and exhibited from inside; it has a passive aspect and an active one. The Schoolmen employed the terms species impressa and expressa to signify these two aspects (impression and reaction) relating sensuous knowledge to the object known or to the subject knowing.

Thomas insists that this sense impression “is not known directly” (id quod cognoscitur). What is present to sense consciousness, what we attain to, is the thing itself — the oak tree. The impression which it produces in me is known only by a reasoning process. We realize why an impulse coming from the external object is the necessary condition by which we know (id quo cognoscitur) — just as nervous activity is needed in sense perceptions and is not perceived by consciousness. Analyzing what actually is, we conclude that something else must be.

The phenomenon, which we have just been considering, is wholly psychical, since it takes place completely in us, and is of a cognitive kind [1]. Therefore, the problem of the transmitting medium of sensations is quite distinct from it. By what medium is it that the oak tree, situated a distance of ten yards, say, from my eye, affects my organism? A few Schoolmen, such as Henry of Ghent, confounded this problem with the previous one. Thomas Aquinas and Duns Scotus, on the contrary, carefully distinguished them. The transmission of the physical action of external objects through the intervening air or water is treated in general in accordance with their notions of physics, which we need not enter into here [2].


  1. The analysis given above deals only with external sensations. In the case of internal sensations, it is the trace left by the external sensation which sets in motion the series of acts of imagination and of sense memory.

  2. Since the species of the Schoolmen are nothing but a vital reaction; since the impulse of the external being (the oak tree) is psychological, it would be a misunderstanding of the scholastic doctrine to consider the species as particles which are detached from the body perceived, and which pass into the percipient. This false interpretation belongs to some decadent Schoolmen of the sixteenth and seventeenth centuries. This fact explains why Leibnitz disparages the scholastic theory of the species. He writes, “Accidents cannot separate themselves from substances nor go about outside of them as the sensible species of the scholastics used to do.” The Monadology, translated by R. Latta, Oxford Press, 1898, p. 129. It is important to notice that the Schoolmen of the decadence, at whom the objections of Leibnitz were aimed, misinterpreted the psychological doctrine of the thirteenth century. Latta does justice to the thirteenth century. “Leibnitz is thinking of a theory (not that of Thomas Aquinas).” p. 220.

The late Dr. Jonathan Dolhenty was the Founder and President of The Center for Applied Philosophy and the Radical Academy, and is Honorary Philosophy Editor at The Moral Liberal. The Moral Liberal has adopted these projects beginning with a republishing and preserving of all of Dr. Dolhenty’s work.