Furthermore we do not even perceive what enters the eye.

The things transmitted are waves or--as Newton thought--minute particles, and the things seen are colours.

Locke met this difficulty by a theory of primary and secondary qualities.

Namely, there are some attributes of the matter which we do perceive.

These are the primary qualities, and there are other things which we perceive, such as colours, which are not attributes of matter, but are perceived by us as if they were such attributes.