Full Idea
A man has passed from mere judgment to expert knowledge of the being of a wagon when he has done so in virtue of having gone over the whole by means of the elements.
Gist of Idea
Expertise is knowledge of the whole by means of the parts
Source
Plato (Theaetetus [c.364 BCE], 207c)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.230
A Reaction
Plato is emphasising that the expert must know the hundred parts of a wagon, and not just the half dozen main components, but here the point is to go over the whole via the parts, and not just list the parts.