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).