Full Idea
The mind cannot perfectly understand things that are even slightly composite unless it considers them a part at a time. ...This is generally called knowing by abstraction. (..the human body, for example).
Gist of Idea
We know by abstraction because we only understand composite things a part at a time
Source
Arnauld / Nicole (Logic (Port-Royal Art of Thinking) [1662], I.5)
Book Reference
Arnauld,A/Nicole,P: 'Logic, or the Art of Thinking (Port-Royal)', ed/tr. Buroker,J.V. [CUP 1996], p.37
A Reaction
This adds the interesting thought that the mind is forced to abstract, rather than abstraction being a luxury extra feature. Knowledge through analysis is knowledge by abstraction. Also a nice linking of abstraction to epistemology.