Is it a technical discussion or a work discussion? In a technical discussion you can certainly say something like "my experience has shown X and Y (without documentation), but if you can show evidence for !X or !Y, I'd take a look". Of course, if both sides are asserting facts without documentation, it's more of a pissing match than a technical discussion; but maybe you'd like to come to agreement on what kind of documentation would be acceptable.
In a work discussion, you can say "well, X or Y may be true, but my experience says !X and !Y, and this is my project, so let's assume !X and !Y, and y'all can blame me later when it becomes apparent I was wrong." and if the other party really doesn't want to go along with it, come up with a limited allotment of time for them to investigate after which either you'll be convinced to change your assumptions or to allocate more time, or they'll go ahead with your assumptions even if they don't like it.