Locke makes a distinction regarding the properties attaching to objects by using two categories: primary and secondary qualities. Both types of quality are capable of producing ideas in us: that is to say that we perceive some aspect of the object by virtue of the quality.

Primary qualities (including "solidity, extension, figure, motion, or rest, and number") are responsible for ideas which "are resemblances of them". In other words, the idea that we have of a primary quality, such as its shape, is to some extent a valid representation of that quality in the object. On the other hand, when we perceive a secondary quality such as colour, then the idea of "colour" is a purely subjective experience that does not in any way represent what the object truly is. Additionally, secondary qualities "depend on those primary qualities": they are nothing more than sensory tricks that we experience as a result of interacting with the object's primary qualities in some way.

At first thought, it might seem that "secondary" qualities are more than just illusions in our minds: after all, a computer can scan an image and accurately report what colours are there, so they must surely exist? But I think that Locke would say that in this case, the computer has in no way had the idea of colour in the same way we would: it has just analyzed various data (which all originate from primary qualities) and reported the synthesis of aspects of that data in a way that corresponds to the labels we give to the idea that is provoked in us by that data.

Imagine a 3-D photocopier which works by scanning an object and constructing a map of all of the sub-atomic particles inside it, and then uses this information to construct an equivalent object. The only information that it needs to capture relates to properties which are primary (how many particles there are, where they are, how they're moving, etc.). It doesn't need to know what colour, or taste, or smell, the object is, because these qualities are all implicit in the primary qualities. For example, an apple "tastes" of an apple because it contains chains of molecules configured in a particular structure: having analysed the layout of the molecules inside these chains, the "taste" secondary quality is captured.