The Enlightenment Period and Empiricism and Rationalism

The Enlightenment period was marked by a revival of interest in determining the nature of reality and knowledge. In the pursuit of this understanding, philosophers expounded ideas that aligned with either of two theories – empiricism or rationalism. John Locke was one of the more prominent philosophers of the time to consider this subject. He subscribed largely to empiricist philosophy, which held that knowledge of reality comes only from experience in perceiving it, that is, knowledge is a representation of reality within the mind, which is shaped by individuals’ unique processing of sensory input coming from real objects. This is in contrast to rationalism, which argues that knowledge comes from logic, which is innate in human cognition. In his Theory of Perception, Locke contends that perception of objects is determined by their properties, which he breaks up into two categories – primary and secondary qualities – and it is this distinction that allows Locke to explain perception in such a way as to provide an argument for his empiricist philosophy.

In Locke’s theory of perception, primary qualities are those that are intrinsic to the object. They are qualities which the object retain regardless of the conditions under which they are perceived, by whom they are perceived, or whether they are perceived or not. He lists them as solidity, extension (that the object occupies space), figure, and motion (or lack thereof). Locke uses the example of division to demonstrate the existence of intrinsic qualities. Citing wheat, he explains that if a grain of wheat is divided, it will still retain the same solidity, extension, figure, and motion.

Locke again uses division to illustrate secondary qualities and the difference between them and primary qualities. Almonds (or rather almond particles), he explains, retain their solidity, extension, figure, and motion when they are divided, and it is this ability to be retained that qualifies these four as primary qualities. On the other hand, their taste, smell, and texture change when they are divided, and it is the fact that division changes them (or general manipulation) that demonstrates that they are secondary qualities. Locke also uses the example of fire to demonstrate heat as a secondary quality. Fire’s heat changes the primary qualities of, for example, wax, by changing its color and figure. Similarly, it induces pain upon contact. However, Locke argues that this heat is not intrinsic to fire, but rather it is its unique interactions with wax and people that gives it this definition of heat. It is wax’s unique property that causes it to melt in reaction to contact with fire, and it is the individual’s perception of heat that results in the idea of pain. If the fire were to make contact with, say, steel, it would not melt due to the steel’s quality, and if it were to make contact with a person with calloused hands, he/she would not feel pain. Locke posits that this variability of perception or interaction based on the perceiver’s qualities makes heat a secondary quality. Thus, he defines secondary qualities as those not in the objects, but rather properties which only exist within our perception.

Phrased another way, secondary qualities are not properties of the objects, but rather the objects’ powers to affect other objects and our perception. The exact nature of these secondary qualities is dependent and variable based on individuals’ unique perception. They change depending on the conditions under which they are perceived and by whom they are perceived. Indeed, the very existence of these secondary properties depends on whether they are perceived or not. To change a primary quality, the properties of the object itself must change; to change a secondary quality, only the conditions under which the object is perceived must change. In addition to heat, Locke lists color, smell, taste, texture, sound, and brightness as examples of secondary qualities. In a sense, he contends that secondary qualities are manifestations of primary qualities that humans and other objects perceive because color, smell, taste, texture, etc. are determined by objects’ primary qualities, namely figure and motion, at the molecular (in Locke’s terms, particles of objects divided to the extent that the parts become insensible) level.

Locke’s division of properties of objects into primary and secondary qualities predicates his empiricist worldview. Essentially, the distinction between primary and secondary properties bolsters his argument that ideas of reality, i.e. knowledge, can only be formed through perception and the inherent modification of the stimuli entailed therein. His contention that all that we perceive are subjective non-intrinsic primary qualities in the form of secondary qualities forms the cornerstone of the subjective experience-based reality that empiricism espouses.