Full Idea
Study things as mathematicians do. Suppose what is not separable to be separable. A man qua man is an indivisible unity, so the arithmetician supposes a man to be an indivisible unity, and investigates the accidental features of man qua indivisible.
Clarification
'qua' means 'simply as'
Gist of Idea
Mathematicians suppose inseparable aspects to be separable, and study them in isolation
Source
Aristotle (Metaphysics [c.324 BCE], 1078a19)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.400
A Reaction
This is the abstractionist view of mathematics. Qua indivisible, a man will have the same properties as a toothbrush. Aristotle clearly intends the method for scientists as well. It strikes me as common sense, but there is a lot of modern caution.