It being impossible to conceive that body should operate on WHAT IT DOES NOT TOUCH (which is all one as to imagine it can operate where it is not), or when it does touch, operate any other way than by motion.
12.

By motions, external, and in our organism.
If then external objects be not united to our minds when they produce ideas therein; and yet we perceive these ORIGINAL qualities in such of them as singly fall under our senses, it is evident that some motion must be thence continued by our nerves, or animal spirits, by some parts of our bodies, to the brains or the seat of sensation, there to produce in our minds the particular ideas we have of them.

And since the extension, figure, number, and motion of bodies of an observable bigness, maybe perceived at a distance by the sight, it is evident some singly imperceptible bodies must come from them; to the eyes, and thereby convey to the brain some motion; which produces these ideas which we have of them in us.
13.

How secondary Qualities produce their ideas.
After the same manner that the ideas of these original qualities are produced in us, we may conceive that the ideas of SECONDARY qualities are also produced, viz.

by the operation of insensible particles on our senses.