Full Idea
Were there no inflexible reality outside of the individual, opposing and limiting it, knowledge of the self and the non-self would never develop.
Gist of Idea
We can only distinguish self from non-self if there is an inflexible external reality
Source
Stephen S. Colvin (The Common-Sense View of Reality [1902], p.140)
Book Reference
-: 'Philosophical Review' [-], p.140
A Reaction
Presumably opponents would have to say that such 'knowledge' is an illusion. This is in no way a conclusive argument, but I approach the problem of realism in quest of the best explanation, and this idea is important evidence.