Posterior Analytics (Bouchier)/Book I/Chapter XXXI

Chapter XXXI: Sense perception cannot give Demonstrative Science

 * No Science can be attained by means of Sensation, which can never prove a universal, though repeated sensations may in time produce a universal, and this a knowledge of the Cause.

Nor can scientific knowledge be gained by means of sense perception, for even though perception may give information concerning a thing’s quality as opposed to its concrete existence, yet an act of perception must indicate the existence of the object in a particular place and at the present time. The universal on the other hand and that which is present in every example of a subject cannot be perceived by the senses, for the universal is not a particular thing visible at the present moment, for then it would not be a universal at all, seeing that we mean by Universal that which is eternal and omnipresent. Since the demonstrations rest on the universal, and universals cannot be perceived by the senses, it is clear that one cannot acquire scientific knowledge by means of sense perception. Even if we could have perceived that a triangle has its angles equal to two right angles, we should certainly have gone on to search for a demonstration of it, and should not, as some assert, have already known the fact by means of perception alone. Perception as an act must deal with the particular alone, while scientific knowledge consists in learning the universal. Thus even if we were on the moon and saw the earth shutting out the light, we should nevertheless be ignorant of the cause of an eclipse. We should indeed see that the moon was being eclipsed at that particular moment, but we should not know the cause of an eclipse in general, for our perception would not be of the universal. I do not deny that after seeing the same phenomenon occur repeatedly we might search out the universal law, and thus attain to demonstration, seeing that knowledge of the universal results from repeated acts of sense perception. But the value of the universal lies in its shewing the cause of particular phenomena, and consequently the universal is more important than the perception of particular cases or the immediate apprehension of such things as have for cause something other than themselves. Of self-caused primaries we are not now speaking. It is then clearly impossible to acquire scientific knowledge of any demonstrable thing, unless the meaning of ‘scientific knowledge acquired through demonstration’ be attached to the phrase ‘act of sense-perception.’

Certain doubtful questions may be solved by a reference to the failure of the sense perceptions. Thus if we had seen certain things we should have made no further enquiry about them, not because we know them simply from seeing them, but because the mere sight of them would have sufficed to give us the universal. E.g. If we saw that the burning-glass was porous and that the light filtered through the apertures, it would be clear why it burns, because we should see the phenomenon occur in every separate glass; but we should yet have to form the abstract idea that this quality is universally true of every possible glass.