Full Idea
If a mental state is broad, then the existence of the mental state entails the existence of its object.
Clarification
'Broad' content gets its meaning from the world, not from the mind
Gist of Idea
Broad content entails the existence of the object of the thought
Source
Tim Crane (Elements of Mind [2001], 1.7)
Book Reference
Crane,Tim: 'Elements of Mind' [OUP 2001], p.27
A Reaction
Hence thinking of non-existent things like unicorns is problematic for externalists. However, externalists can think about numbers or Platonic ideals.