Locke’s epistemology, or his means of supporting justified belief as opposed to conjecture, is very much based on the distinctions he makes as to what most defines elements of reality. As will be seen, all the components of Locke’s thinking are inextricably related to one another, in that ideas and thinking are created by various processes dependent on one another. For example, Locke holds that simple ideas are never initially present in any human mind. What creates them is the necessary quality of actual experience. The individual takes in impressions of sensory natures and these then generate simple ideas, often going to nothing more than cause and effect.

The child getting too close to a fire becomes aware of pain or discomfort, and the simple idea is generated that fire may be dangerous. These are imprints on the mind and of the most elementary forms. Essentially, human beings develop simple ideas through experience of pain, pleasure, or whatever provides distinct sensory input. These imprints are as well necessary for complex ideas to evolve. What occurs is that the human mind constructs connections between simple ideas, and reaches conclusions not provided by experience alone. Returning to the example of the child and fire, it may happen that, while understanding the threat of the fire, the child then perceives that, if properly distanced, the fire is valuable in offering warmth. A conclusion is reached based on multiple sensations combined and promoting more complex thought. This in turn is an example of a simple mode according to Locke, in that a complex idea derives from simple experiences. Relations are also indicated here, as Locke recognizes how aspects of comparison and contrast go to generating complex ideas.

As noted, Locke relies on other elements necessary for the formation of simple and complex ideas. In his epistemology, human beings must accept the distinctions between primary and secondary qualities. The former are known by what may be called substance; concrete realities such as shape and size produce ideas in the mind as to the nature of the thing. The qualities of the physical thing are what generate the idea while, conversely, secondary qualities are more matters of perception. In a common example, the shape of a book is a causal quality, and a property not subject to interpretation. Shape is invariably shape in the external reality. The book’s being blue, however, is a secondary quality because the idea relies upon the individual’s conceiving of the color. This is a quality of sensation, as blueness is not a fixed quality outside of the self. This distinction is critical; Locke is not claiming that blueness does not exist, but rather that the impression of it in the person’s perception exists only as a secondary quality because it is the individual’s idea of blue here that is present.

As Locke’s epistemology is based upon interactive elements, his concepts of primary and secondary qualities exist within a framework of representational reality. In a sense, this is realism defined by how human beings perceive what is before them; a process or distinction is in place between the two. Primary qualities such as weight and shape are not dependent upon any perception of them. They exist whether the thing is experienced or not, whereas secondary qualities cannot exist without the person’s having a sense of what they represent. Subjectivity is within this aspect of representational realism because the perception of the quality of color or some other attribute of a thing is exactly that: perceived. It does happen that these processes of recognizing representational qualities translate to their being primary; a sky as blue, for example, is typically seen as a defining element of it, and consequently a fixed and primary quality. It is nonetheless only a perception. This idea of blueness may be seen differently by others looking at the sky, so it is not “real,” and rather is a representation creating an idea.