CHAPTER III

THE SENSES AND EXTERNAL PERCEPTION

THE problem of external perception comprises two distinct questions. The first is a question of fact, quaestio facti. How, and by what kind of process do we enter into relations with the external world? The second is a question of right, quaestio juris. What do we really know of the external world? The first question belongs to empirical psychology, the second to the criticism of knowledge.

The history of the problem of external perception includes then these two questions which have never been properly separated.

The First Philosophers did not recognize the part which the Subject plays in Knowledge. Sensation explained by the Contact of Like or Contrary Elements.

Even in pre-Socratic philosophy we already find a physiology of the senses, and a crude attempt at an analysis of the knowledge acquired through them. But in order rightly to understand these first attempts, there are two things which it would be well to bear in mind. Firstly, that even those notions which now seem most clear to us were at that time in the human mind still confused and indistinct, like the different parts of an organism in the unity of the germ. Secondly, that, before the Sophists, the part played by the subject in knowledge had not been suspected; it had never occurred to anyone to speculate as to how much of itself the mind may project into a knowledge which presupposes its activity. The prevailing idea in this first period was that sensation is explained by the contact of like elements.

Alcmaeon of Crotona.-Heraclitus and Anaxagoras.-Leucippus and Democritus.

The oldest description of sensible perception that we know of is that of Alcmaeon, a physician of Crotona, a contemporary and perhaps a disciple of Pythagoras. The brain, according to him, is the seat of the soul, and sensations reach it through the medium of channels which start from the organs of sense. We perceive smells when in breathing they reach the brain through the nose. The ear is hollow, and all hollow things resound, therefore the ear resounds when struck by the air in motion: the auditory duct of the ear is the path by which the sound makes its way to the brain. Sight is explained by the reflection of brilliant and transparent bodies, the medium here being the water contained in the eye (Theophr. De Sens). In this theory the quality of the external body passed into the brain, and the problem was to discover the means by which this passage was possible.

According to Heraclitus and Anaxagoras, sensation is not produced by the like, but by the unlike. A consequence of this doctrine was, in the teaching of Heraclitus, that the opposition and union of contraries explain all reality. According to Anaxagoras, there can be no action of like on like, as no change can be produced thereby. Our eyes which reflect objects are obscure bodies. We only feel temperatures which are different from the temperature of our bodies.

The theory of the senses held by Empedocles is part of his general teaching. All bodies have pores (πópoɩ), and moreover there are from every body emanations, effluences (àπодρoaí), so small as to be imperceptible, but which penetrate into the pores of other bodies which correspond to them. All change being caused by mixture or separation, there is no other way of explaining action at a distance. This general law accounts for sensation. Like is known by like, water by water, earth by earth, etc. Hence sensation arises when the particles detached from objects come in contact with the similar parts of the sensorial organs; whether these particles come into contact with similar parts through the pores, or inversely as in visual perception, the similar parts are projected through the pores into external bodies. The diversity of the senses and of sensation is explained by the difference in the pores; each

sense only perceives what is symmetrical with its pores and penetrates into it. The particles that enter the nose or the mouth produce smell and tastes. The air being set in motion penetrates into the auditory duct, "as in a trumpet," and produces sound. The eye is a kind of lantern. Empedocles imagined that he had explained sensation when he had proved the contact of two like elements, one of which belonged to the organism. But on the other hand, in his theories on hearing, and still more in those on sight (relations between two terms), seem to find a faint idea of the role of the subject in sensation.

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In the atomistic hypothesis of Leucippus and Democritus, all our mental images may be reduced to corporeal phenomena (τὰς αἰσθήσεις καὶ τὰς νοήσεις ἑτεροιώσεις εἶναι τοῦ σώματος, Stob. Floril. ed. Mein. IV, 233). Sensations are changes produced in us by external impressions. Since every action of one body upon another originates in an impact, sensation is. itself traceable to a contact or touch, and this contact is in its turn explained by the emanations, which are presupposed in action at a distance. We have representations of things when their emanations reach our bodies, and are diffused all over them (Theophr. De Sens. 54). Only like can act on like, our senses are affected only by things that are similar to them. Emanations become detached from sensible objects without losing their form, and these images (eidwλa), being reflected in the eye, are the cause of vision. Sound is a stream (¿cûμa) of atoms which, flowing from the object, sets the atoms of the air in motion, and when, owing to the symmetry of the elements, this stream of atoms penetrates into the body and comes in contact with the atoms of the soul, sound is produced. Although sounds as well as visible images penetrate the body everywhere, we only hear with our ears and see with our eyes, because these organs are constructed so as to receive the largest quantity of sounds or images and to afford them the most rapid passage.

First Attempts at Criticism. Rational Knowledge opposed to Sensation.-Protagoras: the role of the Subject in Sensible Knowledge.

Side by side with this physiology of the senses, we find

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the earliest attempts at a criticism of sensible knowledge. By the Pythagoreans, by Parmenides, Heraclitus, Anaxagoras, and even by Democritus, true knowledge is contrasted with sensation. To the knowledge derived from the senses Parmenides opposes the unity of Being, Heraclitus absolute plurality, Anaxagoras the chaos, the mixture of corporeal things, and Democritus the impossibility of perceiving the atoms and the void, which, according to him, are the elements of all reality. Still, we must bear in mind that none of these philosophers made any pretence of examining our knowledge of the subject in the light of the laws of subjective thought. Their philosophy was not critical, but dogmatic. In these first attempts at psychology, we also find the distinction between primary and secondary qualities. To Democritus belongs the credit of having first made this distinction. According to him, the qualities of bodies are ultimately reducible to the quantity, magnitude, form, and reciprocal position of the elementary atoms, and they are all derived from the quantitative relations of the atoms. But a distinction. must be drawn between these qualities: some of them, such as weight, hardness, and density, may be immediately deduced from the nature of the atoms themselves; others, as colour, temperature, or sound, depend indeed on the different combinations of the atoms, but only represent the particular way in which we perceive their combination (Theophr. De Sens. 63).

With the Sophists the point of view changes. The relativity of knowledge to the mind is discovered. All is motion, says Protagoras with Heraclitus, but he does away with the absolute reason by which in the teaching of the latter the flux of things is directed. All knowledge is sensation, and every sensation can be traced to the reciprocal action of subject and object, to the impact of their different motions. From this Protagoras infers that there is no reality in sensation, or in sensible qualities; that they only exist one through the other at the moment of the contact of the two phenomena. "Man (ie. the individual man) is the measure of all things" (Plato, Theaetetus, 152 a). That is to say, all things are relative, nothing exists, everything is in a state of becoming. Thus of a newly-discovered truth, scepticism was the first result.

Plato: Physiology of the Senses. Part played by Sensation in Knowledge.

Plato recognizes with Protagoras that sensible qualities result from the relation between subject and object, and that consequently they are a sign, or an expression of reality, not reality itself. The world can act upon the body, which is composed of the same elements as itself. Sensation is only an external impression continuing itself by way of the body into the soul. The diversity in sensible qualities is caused by the diversity in the motions, which the impression communicates to the body, and which the body propagates to the soul (Tim. 43, 64, 75). The sense of touch is all over the body, and gives general sensations (Kowà Ta@nuara), like those of heat, cold, heaviness and lightness, softness and hardness. In every case it is the movement communicated to the corporeal elements which becomes the sensation. The sensation of heat, for instance, arises from the fact that fire, owing to the small size, sharpness, and extreme mobility of its atoms, penetrates into and decomposes the elements of the body. Taste and smell are intermediate senses, by which we ascend to the higher senses of hearing and sight. Sound is the disturbance of the air transmitted by the ear through the brain and the veins to the soul. Plato is always bent on determining the media by which the external motion is propagated to the soul. In vision, the medium is no longer air but light, a kind of fire which is at once in the eye and outside it. The light that radiates from the eye goes out, so to speak, to meet the light radiating from the object. Thus vision is the result of an external motion, which is transmitted, in the first place, to the environing light, then to the light of the eye, and finally to the soul. At night the light of the eye no longer meets the external light, and, the continuity of the transmission being broken, we cannot see (Tim. 45). Since the light belonging to the eye has a part in perception, the latter must have a subjective character. Plato admits and proves this when he shows that the principle of divers visual sensations is contained in the relation between the two lights (the subjective and the objective) on their coming together.

And now, what, in Plato's opinion, is the value of sensible knowledge? He does not deny the reality of space or of