When the mind combines several simple ideas, it forms a complex idea. Locke argues that complex ideas are infinite, but they each fall under one of the following categories: modes, substances, or relations. Modes are abstract rather than concrete, such as the concept of “murder” or “gratitude.” Substances are tangible ideas, such as “man” or “herd.” Relations refer to the comparison of one idea to another.

This chapter is Locke’s first examination of complex ideas. He outlines an intention of holding up a complex idea of a simple mode and examining its various complex iterations. The concept of space is a simple idea that the sensory experiences of sight and touch communicate to the mind. Perception conveys various modes of space, such as measure, body, and extension, as complex ideas. Measurement reveals different modifications of the same concept. Locke suggests that a sense of place is yet another modification of the complex idea of space. All understandings of space come from an object’s relativeness to another.

Simple ideas like motion and solidity inform the mind about space. Some may view concepts like “body” and “extension” as synonymous modifications of the complex idea of space; however, Locke argues that they are distinct.