Full Idea
It is possible to conceive what is not possible.
Gist of Idea
It is possible to conceive what is not possible
Source
Sydney Shoemaker (Causality and Properties [1980], §10)
Book Reference
Shoemaker,Sydney: 'Identity, Cause and Mind' [OUP 2003], p.231
A Reaction
The point here is that, while we cannot clearly conceive the impossible in a world like mathematics, we can conceive of impossible perceptions in the physical world, such as a bonfire burning under water.