Full Idea
The impossibility of seeing two colours simultaneously in a given direction feels like a logical impossibility.
Gist of Idea
Some facts about experience feel like logical necessities
Source
Bertrand Russell (Human Knowledge: its scope and limits [1948], 9)
Book Reference
Russell,Bertrand: 'Human Knowledge' [Routledge 2009], p.113
A Reaction
I presume all necessities feel equally necessary. If we distinguish necessities by what gives rise to them (a view I favour) then how strong they 'feel' will be irrelevant. We can see why Russell is puzzled by the phenomenon, though.