Full Idea
One way to attack a metaphysician would be to enquire from what premises his propositions were deduced. Must he not begin, as other men do, with the evidence of his senses?
Gist of Idea
All propositions (especially 'metaphysics') must begin with the senses
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.1)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.45
A Reaction
This simple idea is the core of empiricism. This is a heavily criticised doctrine, but you must start somewhere. Hume and Russell agreed. Don't forget, though, that Descartes's first move is to reject the senses as untrustworthy.