Full Idea
Men begin to distinguish themselves from animals as soon as they begin to produce their means of subsistence.
Clarification
'Subsistence' is survival
Gist of Idea
Men distinguish themselves from animals when they begin to produce their means of subsistence
Source
K Marx / F Engels (The German Ideology [1846], §1.A)
Book Reference
Marx,K./Engels,F.: 'The German Ideology', ed/tr. Arthur,C.J. [Lawrence and Wishart 1985], p.42
A Reaction
This seems a rather external criterion. Presumably we can ask what biological or mental feature made it possible for men to produce their own means of subsistence, and why it evolved. Darwin puts a different perspective on this idea.