In “Toleration and Intellectual Responsibility”, collected in After The Open Society, Karl Popper rests toleration on three principles:
The first is Socrates’ knowledge of our own ignorance. The second makes rational discussion the aim of free speech: the discovery of truth rather than the incitement of passions or victory in a debate. The third is the one I keep returning to. By talking we may both get nearer the truth even without agreeing, and that is only possible through mutual tolerance, the attitude that what the other person says may at least deserve a hearing. Agreement, Popper adds, is comparatively unimportant in the search for truth, since we may both be mistaken; people agreed for centuries on the Ptolemaic system, and agreement is often the product of fear.
Those three lines are the foundation under the Discourse Protocol. Most bad arguments I have been part of failed the first principle before anyone reached the second.