Full Idea
Did we know the mechanical affections of the particles of rhubarb, hemlock, opium and a man, ...we should be able to tell beforehand that rhubarb will purge, hemlock kill, and opium make a man sleep.
Gist of Idea
If we knew the minute mechanics of hemlock, we could predict that it kills men
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.03.25)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.556
A Reaction
Locke was adamant that we could never know such things, but I take it that we now do know them, and that this is precisely what science aims at. I'm beginning to think that the entire aim of science is to predict nature.