We are ingenious folk! But it’s not clear we need anything as clever, or large, as a Copernican Revolution.

Since we’re on the subject, let’s talk physics. To model how a billiard ball rolls on a table, one makes simplifying assumptions—the table is a perfectly flat plane, the ball is a perfect sphere, the mass is distributed equally, and the like. One does these things not because the simplified model is perfectly true to life, but for three related reasons: (1) the model is often close enough; (2) the math is vastly easier; and (3) you can use it to say things that are more or less true about billiard balls anywhere: here, in Spokane, and—with only minor adjustments—on Mars.

Moral and political philosophy should be like that. They should make simplifying assumptions. They must, if they are to do anything more than reference isolated cases without any extensive explanatory power. And the ability to extend to additional cases is the very reason we do philosophy, at least as a practical matter.

This is why Michael Huemer’s treatment of the non‐​aggression axiom seems like a significant step forward to me. To Huemer, the non‐​aggression principle is not really an axiom at all; it’s a strong but rebuttable presumption. That may seem like a weakening to a lot of folks, but if so they should consider the radicalism of Huemer’s conclusions as evidence that he’s not going weak in the knees.

By contrast, I have to say Zwolinski’s objections sound as if he is indicting libertarianism for saying that the billiard ball was a sphere, while only he knows, sadly, that spheres have nothing to teach us about billiards. Granted, the first was false, but so is the second.

Spheres are a good solid start to the problem, and we ought not to feel too badly about needing refinements from time to time. The word for these refinements is not epicycles, however. It’s engineering.

I think this analogy points the way toward solving many of the conundrums Matt poses in his post. Many of his difficulties, including children, pollution, and the need for a clear definition of property, are all a part of building models that meet particular real‐​world circumstances. Of course abstract theory has much to tell us. And of course it can’t tell us everything.

In one case, though, I will commend to him a useful solution that doesn’t fit the above. His fourth conundrum holds that fraud is unrelated to force, and that a libertarian society might find itself embarrassingly unable to punish fraud. I disagree.

Fraud is wrong because it, like physical force, defeats the will without convincing the intellect. As such: