Full Idea
Some people hold that no inductive argument can give us any reason to believe in the existence of something which could not even in principle be observed.
Gist of Idea
Maybe induction could never prove the existence of something unobservable
Source
A.J. Ayer (The Concept of a Person [1963], §I)
Book Reference
Ayer,A.J.: 'The Concept of a Person etc' [Macmillan 1973], p.87
A Reaction
I see nothing illogical in inferring the existence of a poltergeist from the recurrent flight of objects around my lounge. Only an excessive empiricism (which used to afflict Ayer) could lead to this claim.