We easily fall into the trap of explaining our political, religious, philosophical, etc. differences as caused by the ignorance, laziness, selfishness, stubbornness, or stupidity of those with whom we disagree. Surprise me by showing me someone who never feels this temptation. Some of us try to resist, but none escape the impulse.
I claim that everyone who has a position on some significant idea could find, if they cared to, the name of some person widely regarded as a genius or a saint that opposes them on this particular issue (or once opposed them, if the genius/saint is dead or experienced a change of mind/heart). (Feel free to send me counterexamples, I'd love to receive them.)
In some ideal rational world (Plato's Republic?) we would all be open-minded and look dispassionately at reasons and evidence, and if we disagreed, we would disagree exclusively on values, not on facts or theories. Perhaps, in these circumstances, we would more easily find agreement, and find disagreement less divisive. The saints and geniuses, at least, would all be in agreement in regard to the facts.
We do not live in that world. Debates rarely (never?) change anyone's mind. When we encounter an idea that contradicts our intuition, we ask ourselves "Must I believe this?" Often the answer is no, and any small perceived ambiguity or difficulty in the evidence or reasoning lets us off the hook. In other cases, when presented with arguments that appeal to our intuition, we instead ask "Can I believe it?" and we find that just a crumb of credibility will suffice.
I've stolen this "Can I/Must I" from Jonathan Haidt's book The Righteous Mind. He claims that intuition evolved as a quick and dirty guide to action, and that reasoning evolved to defend the decisions made by the intuition. From this standpoint, our thinking fails to bring us to agreement because we have pushed beyond the boundaries of the circumstances where it evolved. We face decisions about abstract ideas that impact us only indirectly, while our thinking excels at dealing with immediate concrete decisions that impact us and our immediate social group directly.
We could examine the implications of all this, and the difficulties and weaknesses, and perhaps I'll make another blog post on that someday (or you can send suggestions). What I am actually interested in now (finally!) is to wonder about those persons who finally do change their minds. Perhaps it reveals my bias, but I easily thought of some famous right-wingers who repudiated the left: David Horowitz, Whittaker Chambers, Ronald Reagan. John Stuart Mill supposedly moved toward socialism at the end of his life, though I'm not sure whether he repudiated his individualism or merely radicalized it (Fabian society? Karl Marx? or Henry George? Lysander Spooner?) And we cannot overlook Saul's experience on the road to Damascus. Friedrich Hayek? John Stossel? Julian Simon? Bjorn Lomborg? David Ramsay Steele.
I hope my readers will suggest names to add to this list, but even more urgently, names of scholars or scientists who have examined this question of how someone's passionate dedication to one side of a debate moves them to passionately support the opposite.