Refer to extract reprinted in the Reader, p.209 paragraphs 7 and 8.
"To discover the nature of our ideas the better, and to discourse of them intelligibly, it will be convenient to distinguish them as they are ideas or perceptions in our minds; and as they are modifications of matter in the bodies that cause perception in us " Locke, An Essay Concerning Human Understanding, Bk II, Chapter VIII para 7; Reader, page 209.
Summary: Our ideas are not always copies of what is there in the external world.
Locke says it is usually believed that in perception, the ideas that are formed 'are exactly the images and resemblances of something inherent' in whatever it is you are perceiving. (Locke, An Essay Concerning Human Understanding, Bk II, Chapter VIII para 7; Reader, page 209)
The thesis we have been exploring is this: in perception you do not form copies of the thing perceived. Some aspects of the thing perceived are copied in the idea that you form of it, but some aren't.
"Let us give the name of 'sense-data' to the things that are immediately known in sensation: such things as colours, sounds, smells, hardnesses, roughnesses, and so on." Problems of Philosophy, 1st Published 1912, London, Internet edition thanks to the late Professor Makino's Library, Chapter 1.
"...[W]henever we see a colour, we have a sensation of the colour ... The colour is that of which we are immediately aware..." Problems of Philosophy, Chapter 1.
VP