If we reject, as the Empiricists do, the idea that all our knowledge comes from rational principles, we are left with a major question: How can we tell which of our perceptions are real or true? Locke's answer is to suggest the existence of what he calls primary and secondary qualities.

First of all, let us consider an object - a table, for example. Now, Locke's view is that certain qualities of the table are primary qualities of the object (such as the table's shape and size), but others are produced by powers in the object itself, which act on our senses to produce sensations and impressions. Such things as colour, taste and temperature are therefore secondary whilst other primary qualities include number (how many objects there are) and motion (an object's speed or movement).