“Hume’s Guillotine” is a slogan of many faults: the first two being that it’s absurd when applied to itself and that Hume never said it. But a further and perhaps simpler problem is that one sense of “is” means “ought”. When we say that a Boy Scout is always courteous and kind or the Supreme Court interprets the Constitution or human beings are bipedal the “is” predicate or its equivalent is meant to express something that ought to be the case. Were this not so, it would be no deprivation or failure when the description fails to apply. One can not only get an ought from an is, some ises are oughts.

But we can take the description a step further and say that whenever some norm is yoked to something, the norm specifies what is actually the good of the thing. One important consequence of this for philosophers is that when we ask whether an argument is persuasive or not what we are asking is whether it ought to persuade, that is, whether it is true and/or the proper basis for action. There is a logical error in thinking that an argument is not persuasive because we can foresee that it won’t cause people to, in fact, change their minds. What counts as persuasive is determined by the structure and nature of reason and not what we can foresee people accepting, just as what counts as healthy is not what we can foresee people wanting to eat.

I don’t deny the importance of rhetoric or marketing or meeting your opponent where he’s at. But I do object to taking the standard of what is persuasive as anything other than the one that belongs to it properly and first.

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