Thomas Aquinas: How Our Knowledge is Formed


A. Origin of sensations – Psychical and physical aspects
B. Origin of intellectual knowledge

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].

B. Origin of intellectual knowledge

There is a well-known adage of scholastic and thomist psychology, which states that we derive the content of our abstract ideas from the content of our sensations, and, by means of these, ultimately from the material universe. Nihil est in intellectu quod prius non fuerit in sensu. “There is nothing in the mind which was not first in the senses.” Our ideas of life, strength, greatness, motion, action exercised and received, double, half, right, etc. — all these and a thousand others equally abstract in nature — are derived from our sense perception of the objects which surround us. We have proper and direct knowledge of the material world only. Our mind is closely united to our body, and it is in and through the corporeal bodies that we obtain our intellectual knowledge.

It follows from this that even moral ideas (justice, right, etc.) and our knowledge of spiritual beings (the mind, spirits, God) is derived from, and must be expressed in terms of the material, by means of comparison, analogy, negation, and transcendence. We have only an improper and indirect idea of what is spiritual. Although we can prove that there is such a thing as a spiritual being, we do not know in what it consists properly, and our feeble minds have to conceive it by applying to it the notions of being, reality, causality, etc., which have come to us through the channel of our senses.

The problem of the origin of our abstract thoughts, however, is to be solved in the same way in which it is solved for our sensations. But it is more complicated on account of a special difficulty.

Before meeting this difficulty, let us take note of the similarity which exists between the processes of sensation and of thought, and why, in the last analysis, both will be solved in the same way. This similarity consists in the initial impression coming from an external impulse, and followed by a characteristic reaction which belongs to thought as well as to sensation. For, experience and consciousness alike prove that the mind also needs to be determined or completed by the corporeal object known, and that it does not derive merely from itself the content of its ideas. A blind man has no idea of color. Left to itself, our mind would be an empty desert, or a clean slate (tabula rasa), with nothing written on it [3]. Here, as in the case of sensation, there is a passage from potentiality to actuality; there is an initial passive state, and there is an impression which is received (species intelligibilis impressa). The two horses or dollars from which I derive the abstract idea of the number ‘two,’ or of ‘money,’ ‘power,’ ‘form,’ etc., act upon my mind. And just as in the case of sensation, the mind reacts to the stimulus and answers by a vital act, by means of which the phenomenon of knowledge is completed (species intelligibilis expressa).

Now, we have to deal with a special difficulty which arises in the case of abstract knowledge. This difficulty appears because it is necessary to harmonize the doctrine of which we have just been speaking with a central teaching of scholastic metaphysics. We shall see later on that the universe of all Schoolmen without exception is a pluralistic one, and that each of the myriad beings of which it is composed has its own separate and independent existence (VIII, A). Each oak tree possesses its own being, independent of all others, and this is equally true of men, animals, etc. And thence comes the difficulty: a particular individual thing, such as an oak tree, can give rise to a sensation of sight which is in turn particularized; but how can it give rise to abstract notions such as life, cylindrical form, without the particularizing conditions which belong to each real living, or cylindrical being? How can this particular living being give rise to the notion of life as such? How can the concrete be known abstractly?

The complete collection: The Summa Theologica of St. Thomas Aquinas (Five Volumes)

The external object (which we here suppose to exist outside of us) cannot determine thought in the same way as it determines sensation. By itself alone it is powerless. The two horses, being particularized and individual, cannot, by means of the sensations they produce, give rise to an impression in us which gives them a mode of being different in kind and superior (abstract) to that which really belongs to them (particular, concrete). Otherwise we should have a cause producing an effect superior to itself. The less would produce the more. At this point, Scholasticism adopts an Aristotelian theory. It is not only the two horses or two dollars which act upon my intelligence, but the sensation of the two horses or dollars act in cooperation with and in dependence upon a special spiritual power within me, which “shines upon the sense data, and makes them capable and ready to produce a knowledge in which reality is deprived of all its concrete and individual features.” This creative power is called active intellect (intellectus agens), and in opposition to it the mind or the intelligence in which the impression is produced, under the twofold influence of the corporeal beings and the intellectus agens, is called intellectus possibilis.

It is important to note here as in the case of sensation, that our minds grasp directly, in the two dollars, the content ‘two,’ ‘money,’ ‘paper,’ etc.; but in attaining these notions, we are aware neither of the spiritual power of abstracting, nor of the impression (species impressa) which it produces in us by the object known. It is again by a process of reasoning, which seeks for an adequate explanation of the phenomenon, that we pass from what is to what must be. This does not imply that by means of this theory we understand the whole mechanism of thought. The latter remains a mystery. In many questions we must be satisfied to know that something exists, even if we cannot penetrate its inmost nature. We ought never to ask of a theory more than it undertakes to do.


  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.

  3. Summa Theol., Ia, q.79, art. 2.

The System of Thomas Aquinas by Maurice de Wulf edited for the Internet by Jonathan Dolhenty Ph.D. The original copyright of the work by Maurice de Wulf is held in the Public Domain, as edited Copyright © 1992 – 2012 Jonathan Dolhenty and The Moral Liberal.