Full Idea
Among all the ideas we have, as there is none suggested to the mind by more ways, so there is none more simple than that of unity, or one; ..every idea in our understanding, every thought of our minds brings this idea along with it.
Gist of Idea
The idea of 'one' is the simplest, most obvious and most widespread idea
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.16.01)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.205
A Reaction
What does Locke mean by 'suggested' to the mind? I take it that this phenomenon of psychology (or of reality, if you like) is the foundation of mathematics, making one clearly prior to zero.