Full Idea
It seems natural and plausible to say that it is the fact that a necessary truth is itself necessary that determines its truth in all possible worlds. This intension determines its extension across possible worlds.
Gist of Idea
Possible worlds don't fix necessities; intrinsic necessities imply the extension in worlds
Source
David M. Armstrong (Truth and Truthmakers [2004], 08.1)
Book Reference
Armstrong,D.M.: 'Truth and Truthmakers' [CUP 2004], p.95
A Reaction
Well said. To me (but not to Armstrong) this implies essentialism, that the necessity arises from the intrinsic natures of the things involved. The whole Lewisian approach of explaining things by mapping them strikes me as wrong.