Full Idea
If the mental states of others are known only through their behavioral and other outward manifestations, while this is not true of our own mental states, why should we think our own mental states are anything like those of others?
Gist of Idea
If we know other minds through behaviour, but not our own, we should assume they aren't like me
Source
Donald Davidson (Three Varieties of Knowledge [1991], p.207)
Book Reference
Davidson,Donald: 'Subjective, Intersubjective, Objective' [OUP 2001], p.207
A Reaction
His point is that if you seriously doubt other minds, you should follow through on the implications. But that is to treat it as a theory about other minds, rather an a sceptical worry. Descartes didn't walk into walls while writing Meditation 1.