Full Idea
Locke seemed to think the having abstract general ideas is what puts the widest difference in point of understanding betwixt man and beast.
Gist of Idea
For Locke, abstract ideas are our main superiority of understanding over animals
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by George Berkeley - The Principles of Human Knowledge Intro §11
Book Reference
Berkeley,George: 'The Principles of Human Knowledge etc.', ed/tr. Warnock,G.J. [Fontana 1962], p.51
A Reaction
I currently favour meta-thought (thought about thought) as the distinction of homo sapiens, but maybe abstraction is an aspect of that, because you have to pick out common factors amongst a variety of experiences.