Having developed in Book I his argument concerning the nonexistence of innate ideas, Locke undertakes in Book II to describe in detail the process by means of which ideas come to be present in human minds. His fundamental thesis is that experience alone is adequate to account for all the ideas included in anyone's store of knowledge.
In beginning this discussion, he calls attention to the fact that neither the belief in an immortal soul nor the phenomenon of sleep can furnish any evidence for the existence of ideas prior to one's experience. Although the claim has been made by some thinkers that ideas were present in the soul before it was united with the body, he shows that this cannot be the case. His reason is that thinking is an activity which takes place only in bodies, and without thinking there can be no ideas. The same may be said with reference to the phenomenon of sleep. Thinking takes place only when one is awake. If we assume that ideas are present when one is not awake, there would be no way of distinguishing between having ideas and not having them.
All ideas, according to Locke, enter the mind by way of the senses or one's reflection on the materials that have been received that way. The first of these he designates by the term sensation, which refers to the conscious states that are produced by the action of external bodies on the mind. It is in this way that we derive our notions of color, heat, cold, softness, hardness, bitter, sweet, and all the sensible qualities of which one ever becomes aware. Since it refers to the action of external bodies on the mind, it might be called the external sense.
The second source of our ideas is the perception of the operations which take place within one's mind as it assimilates and interprets the materials that have been received through the senses. This includes such processes as thinking, doubting, believing, knowing, willing, and all the various activities of the mind of which we are conscious in understanding ourselves and the world about us. Because this source is within the mind, it might be designated as the internal sense. Locke, however, prefers to use the term reflection instead because he believes this will help to avoid confusion with the external sense or sensation.
Ideas are classified as simple and complex. The simple ones are the particular ones that may be considered singly. Complex ideas are made up of simple ones that must be viewed or taken together. Simple ideas are derived in a number of different ways, but they always refer to a separate and distinct quality that is present in one's mind. It is true that in the objects which are external to the mind, several of these qualities are often combined. For example, we may say of an orange that it is soft, yellow, sweet, and round. Nevertheless, in our minds each of these qualities is separate and distinct.
All simple ideas enter the mind through one of the five senses, and it is impossible to experience sensations of any other kind than those for which the sense organs are adapted. It is conceivable that other qualities may exist in the world around us, but if they do it is impossible for us to know anything about them. In receiving sensations, the mind is passive, which is one of the characteristics of simple ideas.
The situation is different in the case of complex ideas, for these are due in part to the activity of the mind. According to Locke, these are formed in three different ways: combining simple ideas into compound ones, comparing ideas with one another, and abstracting from a number of ideas elements that are common to the members of the group.
There are four ways in which simple ideas may enter the mind. First, they may enter through one sense only. Second, they may enter through more than one sense. Third, they may come from reflection only. Fourth, they may make their appearance through a combination of all the ways of sensation and reflection. Each of these ways may be illustrated in the following manner.
The first group includes ideas of any of the colors, tastes, sounds, or smells that may be experienced. It includes also the sensations belonging to touch such as heat, cold, and solidity. In all of these sensations, there is a wide degree of variations, and we have names for only a comparatively small number of them. Solidity, for example, may be described as that which hinders the approach of two bodies when they move toward one another. It is closely related to the ideas of space and hardness, and yet it is distinct from each of them.
In the second group, we have ideas of objects in which several distinct sense qualities are combined. An example of this can be seen in the idea of a metal, such as gold, which at the same time is bright, yellow, and hard. In fact, most of the objects that we experience have more than one sense quality. In addition to these qualities, we have also the ideas of space, figure, rest, and motion.
In the third group, we have the ideas of perception or thinking, and volition or willing. Some of the different modes in which these ideas are present include remembering, reasoning, judging, knowledge, and faith.
In the fourth group, we have such ideas as pleasure, pain, power, existence, unity, and succession.
We normally think of the ideas in our minds as having been caused by the objects that exist in the outside world. It is true that some of these ideas, such as cold or dark, may refer to the absence instead of the presence of certain qualities, but this does not mean that they have no external cause. Even a negative cause can produce a positive idea.
In discussing the problems that are involved in the development of human knowledge, it is important to bear in mind that what exists in one's consciousness is not identical in every respect with that which exists in the external world. If our ideas did not tell us something about the objects that are outside of our minds, we would have no knowledge of anything pertaining to the world around us, which is something that Locke's theory of knowledge would not permit him to admit.
At the same time, he was convinced that the ideas we do have are caused by external objects, and at least some of the qualities revealed to us through sensation are not only in our minds but are also in the objects to which these qualities refer. It is necessary then to make a clear distinction between those qualities that exist only in our minds and the ones that also belong to the external objects. This is what Locke attempted to do in what he had to say about primary and secondary qualities.
We are told that primary qualities are inseparable from the bodies to which they belong. They include solidity, extension, figure, number, and mobility. Any physical body will possess these qualities no matter how many changes may take place within it or how many times it may be divided into smaller parts. For example, a grain of wheat may be divided into two parts, which in turn may be divided again and so on without limit, but no matter how small the particles into which it is divided may become, they will still possess these same qualities. It is quite true that the particles may be too small to be perceived by the senses, but they still possess size, weight, figure, number, and motion.
Secondary qualities include such items as colors, sounds, tastes, and smells. These exist only in the minds of those who perceive them, although they have been caused by the powers that are present in the primary qualities which do belong to the objects themselves. While it is customary to think of the qualities as existing in the objects rather than in the minds of people, a careful analysis makes it clear that such is not the case. Neither colors nor sounds would ever exist apart from some mind which perceives them. The natural tendency to assign these qualities to external objects is due to the fact that the powers which cause them are too small to be revealed to the senses, and thus it appears that the qualities which are sensed are really in the objects.
Simple ideas include not only the ones that are derived from the senses but also the ones that are derived from the activities of the mind itself. One of these is the idea of perception, which Locke tells us is the first faculty of the mind exercised about our ideas. What perception is can be known only by those who have experienced it and have reflected on the nature of that experience. Impressions can be made on the sense organs, but unless these motions are communicated to the mind, there will be no ideas which enable the one who receives them to understand what they mean. Fire, for example, may burn one's body, but until the sensations are communicated to the mind, there will be no idea of either heat or pain.
The awareness of these ideas is what is meant by perception. Perceptions are present in various degrees, and to some extent they may occur in children even before they are born. They may occur in the so-called lower animals. These facts should not be interpreted as giving support to the belief in innate ideas since in every case the perception is made possible only by means of some external object. The degree of perception that is experienced by normal human beings is one of the characteristics that distinguish the human mind from that of the lower animals.
A further faculty of the mind that makes knowledge possible is memory, or the retention in the mind of ideas that have been experienced in times past. It is this power of the mind that makes contemplation and reasoning possible. The fact of memory does not imply for Locke any notion of a subconscious mind in which ideas are stored and from which they can again be brought into the level of consciousness. Rather it means that the mind has the power to revive perceptions that have occurred before and to do so with the additional perception that it has had them before.
Besides perception and retention, there are other simple ideas that are derived from the activities of the mind. These include discerning and distinguishing between several different ideas. Also included are such ideas as comparing, compounding, naming, and abstracting. It is the extent to which these activities are present that distinguishes normal people from madmen. Locke concludes his discussion of simple ideas with these words:
I pretend not to teach, but to inquire; and therefore cannot but here confess again that external and internal sensations are the only passages I can find of knowledge to the understanding These alone so far as I can discover are the windows by which light is let into this dark room.
In these chapters, Locke has attempted a description of the process by which ideas are formed in human minds. While the source of ideas lies in an external world, any knowledge that one possesses about this source must enter the mind by way of sensation or reflection. Simple ideas are first in the order of appearance in the mind, and it is from these simple ideas that all of the other ones are constructed.
In making this analysis, it seems quite probable that Locke was influenced by the way in which the physical scientists of his day had described the nature and structure of material bodies. They had put forth the view that all physical bodies are composed of atomic particles which are constantly in motion. The differences between various physical bodies could thus be accounted for by the various combinations of these units of matter. Locke's explanation of mental phenomena is a striking parallel to the one given for physical bodies. He tells us that simple ideas derived from either sensation or reflection are the units out of which human knowledge is composed.
This explanation, it should be noted, is not without its difficulties, for it is by no means certain that ideas make their appearance in that order of sequence. Take, for example, the idea of an apple or an orange. It seems quite unlikely that one perceives the particular color, shape, and odor of the object first of all and then proceeds from them to the idea of the object as a whole. When through a process of introspection we examine our own minds, we normally find that the perception of the object as a whole occurs first, and this is followed by an awareness of the color, shape, and odor which belongs with it.
In other words, the sequence appears to be the reverse of what Locke maintained. This, however, is a relatively minor point, to which it might be replied that Locke has not asserted that ideas are always received "in their simplicity," nor has he denied that a simple idea may be in some instances an abstraction from actual experience. What he was most concerned to point out was that simple ideas are incapable of further analysis.
A more serious difficulty arises from the attempt to account for sensations by saying that they are caused by the powers that are present in the qualities which belong to external objects. One may ask on the basis of Locke's theory how it would be possible for one to know that Ideas are caused by anything. From which one of the five senses do we derive the idea of a cause? Obviously, a cause is not something that has color, sound, taste, odor, or feeling. Neither can we say that it is derived from reflection on the sensations that have occurred, for while these sensations appear in a certain order, there is nothing to indicate that they had to occur in that order.
Eventually, empiricists who followed Locke came to the conclusion that causality is a characteristic of minds rather than external objects. Locke did not interpret causality that way. He assumed that it belonged to the world of external objects, for this was something that the scientists of his day had not questioned, and he accepted their view concerning it even though no basis for doing so could be found in the method he was using.
The distinction that Locke made between primary and secondary qualities was another point that gave rise to a series of controversies. He had insisted that such items as size, weight, shape, motion, and number were present in the external objects, whereas color, sound, taste, odor, and feeling exist only in the minds which perceive the objects. He had argued that this distinction was necessary because the so-called primary qualities do not change but remain constant regardless of whether they are being perceived by any minds.
On the other hand, secondary qualities do vary according to the changing conditions that are present in the perceiving minds. For example, the color of an object will vary according to the amount of light in which one sees it, and the sound will vary according to the distance that separates him from the object.
But is this distinction a sound one? Some of Locke's critics insisted that it is not. They called attention to the fact that if variability of the qualities in question is the criterion to be followed, the primary qualities vary as much as the secondary ones even though they do not vary in the same way. The size of an object as it appears in the mind will vary in proportion to the distance from which it is seen as well as the density of the medium through which it is seen. The weight of an object is also variable, for it appears to be heavier if one lifts it when he is tired.
Perhaps the most serious difficulty in this part of Locke's analysis arises from his attempt to explain the way in which the qualities that are present in the external object can produce sensations in a human mind. On this point he appears to waver between two different explanations. One of these is expressed in the view that only like can produce like. On this basis, he must assume that the sensations which are in the mind must be like the qualities in the object. This, he tells us, is what happens in the case of the primary qualities. But this principle does not hold for secondary qualities since these exist only in the perceiving minds. Obviously, another type of explanation must be found for them.
It is in this connection that Locke tells us we can only say that the primary qualities which are in the external objects have the power to produce the sensations which occur in the mind. This is not a very satisfactory explanation, for it ignores entirely the question of how an object which is extended in space can act on a mind or consciousness which is not in space. Other difficulties arise in connection with this problem, and these will become even more apparent in the light of what he has to say about complex ideas.