A mechanic with regard to motion.

To the one light and shade; to the other swift and slow are imagined to be capable of an exact comparison and equality beyond the judgments of the senses.
We may apply the same reasoning to CURVE and RIGHT lines.

Nothing is more apparent to the senses, than the distinction betwixt a curve and a right line; nor are there any ideas we more easily form than the ideas of these objects.

But however easily we may form these ideas, it is impossible to produce any definition of them, which will fix the precise boundaries betwixt them.

When we draw lines upon paper, or any continued surface, there is a certain order, by which the lines run along from one point to another, that they may produce the entire impression of a curve or right line; but this order is perfectly unknown, and nothing is observed but the united appearance.