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Skip to 0 minutes and 15 secondsArmed with the distinctions and definitions introduced in the last clip, let's look at this slightly complicated passage from the great philosopher of the Scottish enlightenment David Hume. In every system of morality, which I have hitherto met, the author proceeds for some time in the ordinary way of reasoning. Makes observations concerning human affairs when all of a sudden I am surprised to find that instead of the usual copulations of propositions, is, and is not, I met with no proposition that is not connected with an ought or an ought not. OK. Stop there. What's Hume's claim so far? Well, he's saying that moral philosophers begin with descriptive statements, but then without warning or explanation switch and start using moral statements.

Skip to 1 minute and 4 secondsThey go from the descriptive terms, is and is not, to the moral terms ought and ought not. OK, carry on, David. David indeed. Mr. Hume to you, thank you very much. Anyway, this change from is and is not to ought and ought not is imperceptible but is, however, the last consequence. For as this ought or ought not expresses some new relation or affirmation, 'tis necessary that the reason should be given, for how this new relation can be a deduction from others, which are entirely different from it. That's not enough. I need to get this turban sorted out. I mean, why am I wearing a turban? Is it a turban? OK, so what's that about?

Skip to 1 minute and 50 secondsHume's claim-- and here he is without the turban-- is that you can't derive and ought conclusion from entirely factual or is premises. Why not? Well, here's an example. One, humans die if you electrocute them. Descriptive. Two, Tim is human. Descriptive. Therefore, three, you ought not electrocute Tim. Moral. That looks like a valid deductive argument, but notice you can accept one and two and not accept three. Premises one and two do not entail three. Suppose you're an executioner somewhere that uses the electric chair and that Tim has been properly convicted of a ghastly capital offence. You might agree with premises one and two. After all, that's why you use the electric chair.

Skip to 2 minutes and 50 secondsAnd you wouldn't be making a logical error if you denied the conclusion. And notice that you don't have to be pro capital punishment to see that that argument is invalid. We might think the executioner or the system is wicked, but that's not part of the evaluation of this argument. Why does this matter? Well, people try to draw moral conclusions from factual premises all the time. Here are some examples. Humans evolved to eat meat. We have canine teeth and so on. Therefore, we ought to eat meat. The natural function of sex is procreation and homosexual behaviour cannot be procreative. Therefore, homosexuality is morally wrong. Some individuals of most species are, by nature, homosexual. Therefore, homosexuality isn't immoral.

Skip to 3 minutes and 51 secondsNone of those arguments are deductively valid. They all purport to draw a moral conclusion from factual or natural premises. They all cross the is ought gap. So one lesson is to be careful. Don't assume that you can draw moral conclusions directly from statements of fact, and watch out for arguments that do. Remember our definition of a moral argument. A moral argument is an argument that includes at least one moral statement. Well, we can also see that if we're to have a valid deductive argument with a moral conclusion, there must be at least one moral statement in the premises to the argument. For instance, humans evolved to eat meat. We ought to live consistently with our evolution.

Skip to 4 minutes and 48 secondsTherefore, we ought to eat meat. Bill Gates is rich. The rich ought to help the poor. Therefore, Bill Gates ought to help the poor. Those arguments are fine so far as validity goes. They avoid the is ought gap because they move from one or more ought premises to an ought conclusion. You can't accept the premises, including the ought premises, without accepting the conclusion. Now we shouldn't assume too quickly that arguments which appear to violate the is ought gap in fact do so. Often, such arguments will have an implicit ought premise. The arguer might not say, rich people ought to help the poor, but perhaps it's obvious that they assume that premise. So remember to be charitable.

Skip to 5 minutes and 41 secondsWe're always interested in responding to the most powerful version of the argument we can identify. We want to avoid the straw man fallacy. Again, discussed in the fallacies document we gave you in week one. And that will often require us to complete an argument we're assessing, in ways that are plausible interpretations of an arguer's uncertain intent and which make their argument as good as it can be. One final thing. Remember that deductibility isn't all there is to it. Think back to John Bishop's remarks about arguments for the existence of God. Once we've made the moral premises clear, we'll have to defend them. And sometimes non deductive strategies might be the way to go.

The Is-Ought Problem

David Hume claimed that you can’t derive an ‘ought’ conclusion from entirely factual or ‘is’ premises. Why not? Well, here’s an example:

  1. humans die if you electrocute them (Descriptive).
  2. Tim is human. (Descriptive). Therefore,
  3. you ought not electrocute Tim. (Moral).

That looks like a valid deductive argument, but notice that premises one and two do not entail the conclusion, three. Suppose you’re an executioner somewhere that uses the electric chair and that Tim has been properly convicted of a ghastly capital offence. You might agree with premises one and two. After all, that’s why you use the electric chair. And you wouldn’t be making a logical error if you denied the conclusion.

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Logical and Critical Thinking

The University of Auckland

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