I've read this in D'Souza's book What's So Great About Christianity, and I've also heard Ravi Zacharias talk about this, and I got a laugh both times.
One of David Hume's popular philosophical statements was "If we take in our hand any volume of divinity or school of metaphysics, let us ask these questions
- Does it contain any abstract reasoning concerning quantity or reasoning?
- Does it contain any experimental reasoning concerning matter of fact or existence?
If the answer to both is "no," commit it to the flames for it can be nothing but sophistry and illusion."
He is saying, take a book in your hands and ask if it is being mathematically reasoned through, if it isn't--commit it to the flames. Or, if a statement isn't mathematical or scientific it is sophistry and illusion.
But, Hume's statement is a philosophical test for meaning that fails its own test. His statement does not contain any abstract reasoning concerning quantity or reasoning, nor does it contain any experimental reasoning concerning matter of fact or existence.