Many people think that a hallmark of rationality is agreement. They think that genuinely rational inquiries should result in one side convincing the others to agree that its position is best. When agreement cannot be achieved, the matter in question is just a matter of opinion or preference.
This view is behind a commonly heard criticism of philosophy. Philosophy is merely a bunch of opinions or preferences because the history of philosophy is a history of disagreements. Surely, so goes the criticism, if anybody indeed understood anything substantive, then he or she could lay out the reasons, evidence, and arguments that would convince other rational thinkers. Nobody can lay out what convinces. Therefore, nobody has understood anything substantive (modus tollens). This view is also behind commonly held beliefs that politics, morality, aesthetics, religious faith and practice, and much more are nothing more than matters of opinion or preference.
But is it correct? Should we expect agreement to be the norm among thinking people? One tricksy way of showing that the view of rationality as leading to agreement has problems is by pointing out that many people (like myself) do not think that agreement is a hallmark of rationality. They disagree. As a result, the view, if it follows its own rule, becomes merely a matter of opinion or preference.
A less tricksy, somewhat helpful answer distinguishes between maximal rationality and everything less. Regularly reason goes off the rails, and when it does, disagreements among people naturally arise. Feelings and interests can skew rational judgment, just as biases and prejudices can. Poor education and bad mental habits can enfeeble rational thinking. Moral turpitude routinely spills over into thought and judgment. So also if enough goes wrong, minds can simply break. When a couple of these are added to the various shapes and sizes that native intellectual capacity and ability take, they form a toxic stew that should surprise nobody in its failure to produce agreement.
But what about in their best moments when people are maximally rational? Should we not expect agreement then? Probably not. If human reason is not all-powerful and if the world is as big and complex as it seems to be, then we should not be surprised that people even at their best do not reach the same conclusions in speculative matters. Even truths as simple such as 2 + 3 = 5 become horribly convoluted when one presses harder on what exactly are numbers, what is addition, and in what sense are 2 + 3 equal to 5. The Hindu story of the blind men touching different parts of an elephant and then self-reporting wildly different accounts of what an elephant is may be a rather good image after all of the difficulty of reaching agreement in speculative matters.
Similarly, if there are a multitude of genuinely satisfying human goods as there seems to be and if they can be realized in different degrees by means of many, different, concrete projects and plans, then we should not be surprised that people even at their best do not reach the same conclusions in practical matters. Betty wants to climb all of the fourteeners in the contiguous US, whereas Ron wants to raise children, garden, and bake bread. Even if they are broad-minded enough to see the value of each other’s goals, they still disagree so much on what is worth pursuing here and now that they have little hope of joining in the collaborative efforts that is the stuff of friendship and marriages. And so it goes. Juwan wants Paramount Pictures to make more Star Trek movies, whereas Deirdre wants them to make more Mission Impossible movies. Nation R wants justice that settles old scores first, and Nation P wants justice that wipes the slate clean and begins anew. You want ObamaCare, and I do not. And on and on.
In the end, people disagree because they have good reasons to disagree. Thus, we should expect disagreement because disagreement typifies human rationality at its best.
Of course, if you disagree with the above, you prove the point. And if you agree with it, you prove the point again!