Full Idea
Until a baseline has been established by communication with someone else, there is no point is saying one's own thoughts have a propositional content. Hence knowledge of another mind is essential all thought and all knowledge.
Gist of Idea
Content of thought is established through communication, so knowledge needs other minds
Source
Donald Davidson (Three Varieties of Knowledge [1991], p.213)
Book Reference
Davidson,Donald: 'Subjective, Intersubjective, Objective' [OUP 2001], p.213
A Reaction
This really is building a skyscraper on the slightly shaky claims of the Private Language Argument (e.g. Idea 4158). Animals are so important in discussions of this kind. Is an albatross more or less devoid of thought and belief?
Related Idea
Idea 4158 An 'inner process' stands in need of outward criteria [Wittgenstein]