The use of the term tabula rasa, or "blank slate," to refer to Locke's conception of the mind can be slightly misleading. Locke regards the mind as being initially free of content. One is not born knowing anything. Over time, the mind acquires content from experience, in the forms of both perception and thought.

However, the mind does have the ability to process this content once it is acquired. This ability to create complex ideas from simple data is an innate property of the mind, which is not shared by a blank slate. If Locke had been familiar with the concept of a computer, he might well have said that the mind is a computer which has been programmed to deal with data, in the form of experience, in certain ways. It is experience that provides the content of the mind.

Primary qualities, according to Locke, are those that are intrinsic to an object and unrelated to the person perceiving it. These are qualities such as shape, size, and number. Secondary qualities are less precise and are dependent on perception. These qualities include color, taste, and temperature. Therefore, if two people see a red triangle, there will be no room for dispute about its primary quality (being triangular). The triangle's redness, however, depends on the perceiver. One of them might see that it is closer to being orange. Similarly, if the temperature is seventy degrees Fahrenheit, the number seventy is a primary quality, whereas whether this counts as being hot or cold is a secondary quality.