Far be it from me to divide humankind in two, but were I so inclined, I’d divide it into those who love reason and those who are indifferent if not outright hostile to it. Members of the first group adore the reasoning process and their own reasoning faculties. The others find the process burdensome and discomforting, something that threatens long-held beliefs and intuitions. When I say the members of the first group adore their own reasoning faculties, I do not mean that they are arrogantly confident in their intelligence or immunity from error. Quite the contrary: a Spinozist love of reason contains within it humility, doubt, an awareness of one’s limits and fallibility, and a recognition of the inherently social nature of reason (and language) and the growth of knowledge.

The thing to read in this regard is John Stuart Mill’s On Liberty, a paean to the free and competitive marketplace of ideas. Mill wished to establish that this marketplace was indispensable to learning or at least to approaching the truth. My favorite line, which admirably summarizes most of the little book, is this: “He who knows only his own side of the case, knows little of that.”

Taking that proposition to heart puts one in the right frame of mind to engage in argument. It’s tempting to approach an argument like a high-school debater: I have a proposition to defend, and, damn it, I intend to do just that. This need not imply a willingness to lie or to make dubious moves; rather, it merely implies an overinvestment in the proposition, a sense that, if I lose the argument, I have lost something big, something like a piece of myself. This is understandable. Beliefs form a worldview; a belief shaken is a worldview shaken, and that’s not easy to take. Losing could also mean being or feeling obligated to do things I would rather not do or stop doing things I’m fond of doing.

But, in my view, that’s a bad attitude. I try to think of argument — the comparing of one’s understanding of things with that of others and the readiness to make adjustments as necessary — the way I think of trade: both sides gain no matter how the interaction comes out. (Think of what John Stossel calls the “double thank you” moment that occurs at the store checkout counter.) How can that be?

Mill’s sentence tells us. If you “win” the argument — and you can do this even if the interlocutor doesn’t seem convinced — you will likely have learned more about your own position simply by hearing it criticized. Being required to answer counterarguments will prompt you to plumb the depths of the topic you’re exploring, and you are likely to think of things you might never have thought of otherwise. That’s good! You’ll know your own position better because you know at least some arguments against it. Since you don’t know whether other counterarguments exist, you can look forward to the next intellectual joust as an opportunity to find out.

On the other hand, if you “lose” the argument, you still gain because you have shed an erroneous belief and are now closer to acquiring knowledge that you lacked before the argument. That’s good too.

It’s win-win, just like trade.

I’m not saying the process is one of unmitigated joy. We human beings naturally become attached to our beliefs, intuitions, and conclusions. We can develop a proprietary interest in them. As a result, we are not eager to see them rendered worthless. The reasonable person is not one who never feels that attachment but rather one who puts the attachment aside for the sake of learning. Like an Aristotelian virtue, openness to intellectual challenge can become second nature as one strives to make a habit of it. Practice makes virtue, and discomfort fades.

It’s no coincidence that argument resembles trade: it’s a form of trade, even if it doesn’t always feel like one. The marketplace of ideas is like the marketplace of goods and services. (Of course, access to an idea can be a marketplace good.) In both cases, people assert propositions — goods embody propositions — and they’ll find out whether better alternatives are available. In the commercial marketplace, sellers present their case that their goods at the asking price offer the best way for potential buyers to accomplish their objectives. Competing sellers make counterarguments. Prospective buyers weigh the arguments, looking for flaws. Thus the epistemological case for a free market in goods and services is identical to the case for a free market in ideas. We learn important things about how to flourish that we would likely otherwise not learn. (This was Ludwig von Mises’s and F. A. Hayek’s argument against central economic planning.)

Finally, the libertarian philosophy of full individual liberty — which includes the right to justly acquired material objects — embodies the love of reason as I’ve described it. The libertarian ethic — the nonaggression principle or, as I prefer, obligation — holds that, if you deal with others, you ought to deal with them through reason, not just for their sake but for your own. Persuasion is the opposite of force, though I acknowledge that some people’s discomfort with reason stems from their conflating the metaphorical compulsion of a good argument with the actual compulsion of a government command. The libertarian philosophy embraces Athens — reason and persuasion — over Jerusalem — revelation and commandment.

I think this provides a case for the free society that is in a sense Cartesian. Descartes of course wrote that one can doubt everything except the existence of doubt and the doubter. (I’m not saying I agree with Descartes.) Applying something like this method to ethics and politics, we may say that, while one may reasonably doubt propositions about how society ought to be constituted, one cannot reasonably doubt the value of doubt and thus the freedom to doubt.

So stated, my proposition might win something broad assent, so I’ll push it further. If one should have the freedom to doubt — call it the right to doubt — then one should also have the right to express doubt. Expressing it is necessary to ascertain if it is reasonable. And if one has the right to express doubt, one has the right to acquire the physical means of maintaining one’s life and of expressing doubt. I’m using right to mean a valid claim to be free from aggressive force and to defend against such aggression, so naturally one’s exercise of this right cannot entail the use of aggressive of force against others, who also have the right. Needless to say, respect for such rights will generate a variety of humane institutions.

Any doubters out there?