Full Idea
Scepticism about the external world is possible because you can never build existence into the appearances, so it must always be inferred or assumed.
Gist of Idea
Scepticism about reality is possible because existence isn't part of appearances
Source
Colin McGinn (Logical Properties [2000], Ch.2)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.45
A Reaction
When McGinn's claim that existence is a very universal property begins to produce interesting observations like this, I think we should take it very seriously.