In “On Toleration”, collected in After The Open Society, Karl Popper rests toleration on three principles. I keep them close:
The first is Socrates’ knowledge of our own ignorance. The second makes rational discussion the point of free speech. The aim is to find the truth, not to stir feelings or win a debate.
The third is the one I keep returning to. By talking we may both get nearer the truth even without agreeing. That is only possible through mutual tolerance, the attitude that what the other person says may at least deserve a hearing. Agreement, Popper adds, matters little in the search for truth, since we may both be wrong. People agreed for centuries on the Ptolemaic system. 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.