Full Idea
To say that contents of my belief are narrow is to say that they are intrinsic to me, hence that any perfect twin of mine would have beliefs with the same contents.
Gist of Idea
If content is narrow, my perfect twin shares my concepts
Source
Gabriel M.A. Segal (A Slim Book about Narrow Content [2000], 5)
Book Reference
Segal,Gabriel M.A.: 'A Slim Book about Narrow Content' [MIT 2000], p.123
A Reaction
I personally find this more congenial than externalism. If my twin and I studied chemistry, we would reach identical conclusions about water, as long as we remained perfect twins.