Wondering whether the world and other persons exist.

Fundamentalists are notoriously difficult to have conversations with because they have a world view that allows them to dismiss inconvenient facts and arguments.  There is, however, a way to debate fundamentalist positions that adroitly avoids this difficulty.  

The strategy, sometimes referred to as 'a transcendental route to a conclusion', is derived from Immanuel Kant's transcendental deduction, as set out in  'The Critique of Pure Reason.

The argument is sometimes deployed against skeptics wondering whether the world and other persons exist.  The fact that the question can be asked involves language, and languages could not exist without communities of language users.