Version 2, November 1, 2005
The modern philosophers rejected the Aristotelian description of human beings and physical objects in favor of new ways of describing them. The physical world was now to be described in a mechanical way. The human body, with its organs of sense, was itself treated as a machine. This opened up the question of how the knowing mind should be described.
Hobbes adopted a materialism that describes the mind in terms of corpuscles and their motions. Descartes and his followers held that the mind is an immaterial substance which exists independently of the physical world. Others like Locke adopted a description of the operations of the mind which did not make any pronouncements about how the mind is specifically related to the physical world. The most radical position, which emerged in quite different forms with Berkeley and Kant, held that the physical world itself is dependent on the mind.
It was generally held that the human mind possesses several cognitive faculties which are productive of knowledge. Reason (or understanding, or intellect) is the faculty which produces knowledge and opinion through inference. According to some, it also produces knowledge through rational intuition. Sense-perception provides us with information about the external world, and introspection shows us what is going on in our own minds. Memory allows for the recovery of information that was originally garnered from intuition, inference, sense-perception and introspection. Another faculty of importance is imagination, though its role in the production of knowledge was quite controversial.
An important non-cognitive faculty that enters into the various accounts of knowledge is the will. Its most important role is in assent. Most of the philosophers of the period held that we exercise our volition when we assent to the truth of propositions, though it may also be that in some range of cases assent is induced willy-nilly. Freedom of the will in assenting allows the ascription of epistemic responsibility, which was described at the end of the last module.
All of these faculties must have something to operate upon. The ancient Greeks called this something the phantasia (presentation, impression). Descartes early on in the modern period introduced the word "ideas" to denote the objects of our mental operations. Perhaps the most famous definition of the term is due to Locke.
It being that term which, I think, serves best to stand for whatsoever is the object of the understanding when a man thinks, I have used it to express whatever is meant by phantasm, notion, species, or whatever it is which the mind can be employed about in thinking; and I could not avoid frequently using it. (An Essay concerning Human Understanding, Introduction, Section 8)There were different views about the original sources of ideas. Sense-perception and introspection were the only two sources allowed by the empiricists. The rationalists added to this rational intuition. The ideas grasped in intuition might be innate, was with Descartes, or it might be external, as in Malebranche's claim that we intuit directly the content of God's mind. It is controversial how the rationalists understood innate ideas, and we will not discuss the matter further here (except to note that Locke vigorously criticized the doctrine and Leibniz defended it against Locke's criticism). Nor will we investigate the intriguing notion that we can have direct insight into a higher metaphysical realm.
Descartes was careful to distinguish between the perceptual idea as it is in the mind and the bodily states which cause it. He thus defined sense as a wholly mental process whose product is a seeming to perceive (Meditation II). This generated the epistemological problem, to be discussed below, of determining which of the ideas of sense really come from objects and which are the production of the imagination.
Locke muddied the waters considerably. He introduced an ambiguity in the word 'idea.' One usage was that of Descartes: the idea is the "immediate" object of the mind's operations. The other is one Descartes would have heartily rejected: the idea is the whatever it is that the immediate idea represents (An Essay concerning Human Understanding, Book II, Chapter 8, Section 7). Thus Locke often called "ideas" what Descartes called "qualities."
The Description of Perceptual Knowledge
The Aristotelian tradition which was dominant before the modern period allowed that human beings have knowledge of physical objects through perception. Most modern philosophers carried on in this tradition, although there were a number of exceptions. They wanted to hold that people know that physical objects exist and that they have various properties. But throughout the period there was also considerable skepticism about both these claims.
The treatment of perceptual knowledge by the modern philosophers generally begins with a description of the process of perception. Perception arises through a physical interaction between objects external to the body and the surface of the body, leading to some alteration in its internal parts. The predominant way of explaining this interaction was corpuscularian. The outer surfaces of the human are bombarded by tiny particles which are or had been in contact with the external body (the former in the case of touch, the latter in the case of the other sense-modalities). This gives rise to motions within the human body, culminating in some bodily state. This bodily state in turn is supposed to give rise to the idea in the mind.
As with the ancients, the question for the moderns is whether the information found in the perception corresponds to the real character of the external bodies. The ancient skeptics had pointed out the relativity of the perception to the situation in which the perception occurs. This fact had to be dealt with by the modern philosophers as well.
Moreover, they were faced with the problem that it is hard to see why the complex physical process that produces perception should represent the object as it really is. This is in contrast to the crude Stoic "impression" account of perception or the Aristotelian account of forms passing from the object to the mind. This may be why the ancient atomist Democritus thought that we cannot gain any knowledge through the senses.
As will be seen, the modern philosophers felt compelled by skeptical problems to deny that we can have perceptual knowledge through perception alone. What is required is further processing of the information by the intellect.
Primary and Secondary Qualities
The relativity in the perception of color, temperature, taste, and many other qualities led the ancient skeptics to the general conclusion that all assertions about objects based on sense-perception are doubtful. But is this generalization justified? Perhaps there are some qualities of objects which can be known despite the relativity in their perception.
On corpuscularian accounts of nature, the particles which make up bodies have qualities such as size, shape, resistance to penetration, motion or rest. These qualities are the objects of scientific investigation and they can be modeled mathematically. The special status of such qualities had been noted by the ancient atomists, as well as by Galileo, Descartes, and Locke at the beginning of the modern period. Locke called these qualities "primary." As he described them, primary qualities are those without which we cannot conceive physical objects at all.
Perceptible bodies made up of the corpuscles have qualities which are aggregated from the qualities of the particles themselves. The perceived shape of a body is not the same as the shapes of its physical components, but it is nonetheless real, and the same holds for all the rest of the qualities we have mentioned.
Now it may be that because of relativity, we are unable to determine with perfect exactness what the size, shape, etc. of a gross perceivable body is. But we can make rough judgments. I might know, for example, that a golf ball is roughly spherical and that it is not even close to being a cube.
It must be recognized that any perceptual knowledge of these qualities of bodies may require the use of reason. This is due to the relativity built into the perception of primary qualities. Thus, if a tower looks round in a distance and square at close quarters, my purported knowledge that it is square involves a rational judgment that one appearance of the tower is more accurate than another. This judgment might be made, for example, on the basis of my feeling the edges of the tower and noting that there are four roughly straight edges and four corners.
Another way in which the (rough) physical qualities of a body may be known is on the basis of a mathematical account of perception. Early modern philosophers such as Descartes and Hobbes tried to provide such accounts, which would explain why an object appears as it does when observed under different conditions. This rational explanation of relativity diminishes or eliminates its threat to the accuracy of our perceptions.
Left out of the corpuscularian model of physical objects are such qualities as taste, smell, sound, and color. They are not necessary to the understanding of the properties of physical objects. The perceptions which represent objects as sweet, smoky, loud, or red are explained in terms of the interaction of corpuscles with our bodies. So what is represented is not qualities of bodies as such, but rather the end products of the interactions between those bodies and our perceptual mechanisms.
Locke called those qualities which are entirely relative to perception "secondary." The relativity arguments seem to apply conclusively to the secondary qualities. There is no fact of the matter regarding whether an object taken by itself is hot or cold, for example, but only a fact about the effect of the agitation of the particles making up the object on our perceptual faculties.
We will consider here an objection to the distinction between primary and secondary qualities, due to George Berkeley, writing in the eighteenth century. He claimed that relativity considerations apply to primary qualities as well as to secondary qualities. He thought that if he could break down the distinction, he could re-establish common-sense knowledge claims such as that a rose is really colored red. (This despite the relativity of our perceptions of all qualities. How he tried to do this is discussed below.)
It has already been noted that there is relativity in the perception of primary qualities. So the real question is whether the relativity of the perception of some qualities can be overcome in a way that it cannot with respect to other qualities. I have suggested that conformity of tactile perception to visual perception is one way. But Berkeley could appeal to the fact that the two are sometimes in conflict, as is the case with the stick in the water, which is seen as crooked but felt as straight.
The second way of overcoming relativity that has been noted is through the use of a theory of perception. Berkeley himself thought he had refuted Descartes's geometrical account of vision, which would have explained how our perceptions of size, shape, etc. vary systematically in a way that allows us to judge (roughly) the true size, shape, and so forth of physical objects. But this does not preclude there being some other accounts of perception which would allow us to make correct judgments.
For the moderns, knowledge of the qualities of bodies does not arise from perception alone. Because of the requirement of certainty in what we know, reason plays an essential role in the creation of knowledge. The raw perceptions of sense do not of themselves provide sufficient information to enable us to be certain of the qualities they represent. And of course, we must rely heavily on reason for our knowledge (if any there be) of qualities of objects which are not present to the body.
Knowledge of the Existence of Objects
The preceding discussion assumed that we know that physical objects exist and need to settle which of their properties are known. We must ask exactly how we get into a position to know of a given perception that what it presents as real is in fact real. When we dream, we often entertain perceptions which are of our own manufacture. We may think that we recall having perceived an object which in reality we did not perceive. This is perhaps the most important epistemological issue facing the modern philosophers.
There were two standard answers given to this question. The first was that there is a difference in the appearance of truly representative ideas and those which are of our own creation. The former are described as "livelier" and "more vivid" than the latter. Hume and others pointed out that there are some dull perceptions and lively imaginings which make the character of ideas inadequate as a measure of their success in representing objects.
The second feature is the relation of ideas to one another. Hume in the Treatise (Part I, Book I, Section II) described the most important of these as being constancy and coherence. The same (or resembling) ideas return again and again, and they do so in a very regular way. Normally, we assent to claims about the physical world automatically when the ideas present themselves as lively and well-connected. If there is any doubt, however, we an employ our reason to discover whether the ideas are coherent enough to warrant our assent.
Descartes made a similar claim in the Sixth Meditation. He dismissed the skeptical hypothesis that we cannot tell the difference between waking and sleeping on the grounds that we are made by God so that we are not deceived about the external world when we can connect our perceptions with the whole of our lives. This requires not only perception and memory, but reason as well. Moreover, certainty is attained only when the mind has canvassed its perceptions, its memories, and its intellect to check for conflicting reports.
So as with knowledge of the qualities of bodies, knowledge of their very existence is not secured by perception alone, but by perceptions which are harmonized by memory and reason. Not only are perceptual "ideas" purely "mental," but so were memory and reason, according to most of the modern philosophers (Hobbes being the exception). Because these psychological factors in the generation of knowledge were not describable by mechanical operations of bodies, there was a discontinuity in the explanation of the production of perceptual knowledge. Even if the mind is described as a kind of psychological machine, as with Hume, it operates according to its own laws, and this opens up the prospect for skepticism.
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