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. 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. 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. 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. Fourth, they may make their appearance through a combination of all the ways of sensation and reflection. 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. 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.