Full Idea
It is currently held that we are committed to a belief in the existence of anything over which we quantify.
Clarification
'Quantifying over' something is asserting that there is at least one instance with the relevant property
Gist of Idea
It is currently held that quantifying over something implies belief in its existence
Source
A.J. Ayer (The Central Questions of Philosophy [1973], IX.C)
Book Reference
Ayer,A.J.: 'The Central Questions of Philosophy' [Penguin 1976], p.204
A Reaction
In a fairy tale we quantify over fairies, so some further criterion will obviously be needed.