Is/Ought Fallacy

86 Is/Ought Fallacy

Mark T. Nelson

When considering what we ought to do, our reflections lead us eventually to a consideration of what we in fact do; this is inescapable, for a catalogue of our considered intuitive judgments on what we ought to do is both a compendium of what we do think, and a shining example (by our lights – what else?) of how we ought to think.

Daniel Dennett, The Intentional Stance

Some people think that Daniel Dennett must be wrong here because no good arguments exist that lead from factual premises to moral conclusions. This is the doctrine of the is/ought fallacy (IOF), summed up in the slogan, “You can’t get an ‘ought’ from an ‘is’,” meaning that any attempt to infer a normative conclusion about what we ought to do from descriptive premises about what is the case must go wrong somehow.

This issue matters for two opposing reasons. First, it is natural to think that our moral beliefs need to be based on facts if they are to be more than mere prejudices or moves in a power game. If we can’t infer “ought” from “is,” it is difficult to see how ethics can have this factual basis. Second, certain kinds of is‐to‐ought arguments can be used to support the status quo in objectionable ways: “This is the way things have always been; therefore this is the way things ought to be.”

Some philosophers think that no good arguments exist that lead from “is” to “ought” because no logically valid inferences exist from descriptive propositions to normative propositions, an idea they trace back to David Hume’s (1978) remark that:

In every system of morality, which I have hitherto met with, I have always remark’d, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz’d to find, that instead of the usual copulations of propositions, is and is not , I meet with no proposition that is not connected with an ought , or an ought not . This change is imperceptible; but is, however, of the last consequence. For as this ought or ought not , expresses some new relation or affirmation, ‘tis necessary that it should be observ’d and explain’d; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation be a deduction from others, which are entirely different from it. (469)

Others dispute this reading of Hume, noting that he does not actually say that it is impossible to make such deductions, only that it seems inconceivable unless we can give an explanation of how we can make them (which they say, he goes on to give). Still others note that even if Hume is claiming here that there can be no logically valid inferences from descriptive premises to normative conclusions, it is not clear what his argument for this claim is or that he argues for it at all. Indeed, the doctrine of the IOF is rarely argued for; it is usually just asserted without argumentation.

Charles Pigden (1991) argues for the illegitimacy of the move from is to ought, saying that we should understand Hume as pointing to the conservativeness of logic. According to Pigden, logic is conservative in that “the conclusions of a valid inference are contained within the premises. You don’t get out what you haven’t put in” (423). The implication for morality is this: logically valid arguments will not yield moral propositions as their conclusions if they do not already contain at least one moral proposition among their premises. This raises the question: What is it to be a moral (or other normative) proposition?

If being a moral proposition is just a matter of containing moral words such as “right,” “wrong,” or “ought,” then valid inferences need not be conservative, as the following example from Arthur Prior (1960) shows:

1. Tea‐drinking is common in England.
2. Therefore, tea‐drinking is common in England or all New Zealanders ought to be shot.

In this argument, the premise does not contain any moral words, the conclusion does contain a moral word, and the argument is valid because it is an instance of the “disjunction introduction” rule of inference. Even so, most people feel that it cannot be so easy to get a real “ought” from an “is.” Because the conclusion is a disjunction, we could substitute any arbitrary proposition for the right‐hand disjunct – the moral part of it – without affecting the validity of the argument. This suggests that its conclusion is not a genuine moral proposition at all. Pigden (1991) calls propositions like this “vacuously” moral and qualifies his principle accordingly: “The conservativeness of logic becomes the claim that no (non‐logical) expression can occur non‐vacuously in the conclusion of a valid inference unless it appears in the premises […]. The autonomy of ethics is simply the moral incarnation of this claim – no non‐vacuous ‘ought’ from ‘is’” (424).

What is it to be a moral proposition, then, if it is not just a matter of containing moral words? Stephen Maitzen (1998) and Mark Nelson (1995) have defined genuine moral propositions as all and only those that carry moral commitment, that is, that entail that some particular person is virtuous or vicious, or some particular action is right or wrong, or some particular type of action is required or forbidden. This enables us to explain why Prior’s argument is not a counter‐example to the doctrine of the IOF. Because it is a disjunction, the conclusion of Prior’s argument carries no moral commitment, which implies that it is not a moral proposition at all. Given their definition, however, it is still possible to formulate other counter‐examples to the doctrine of the IOF:

1. “Bertie morally ought to marry Madeline” is one of Aunt Dahlia’s beliefs.
2. All of Aunt Dahlia’s beliefs are true.
3. Therefore, Bertie morally ought to marry Madeline.

Both premises in this argument are descriptive: (1) is a proposition about what some person believes, and (2) is a proposition about the truth‐value of that person’s beliefs. The conclusion is a moral proposition, entailing that a particular person has a particular moral obligation. The argument is valid according to the standard definition of logical validity: necessarily, if its premises are true, then its conclusion is true. It is also compatible with the conservativeness of logic since no terms appear in the conclusion that do not also appear in the premises. An “ought” appears in premise (1), but that is not enough to make it a normative premise since the word is mentioned in a report of someone’s belief and is not used in a normative proposition as such. The truth or falsity of the premises is irrelevant since that does not affect the validity of the argument, and we are currently considering the doctrine of the IOF as a claim about logical validity.

Faced with such counter‐examples, some philosophers conclude that the IOF is not a logical problem but an epistemological one, meaning that even if inferences like this one are logically valid, they cannot be used epistemologically to warrant anyone’s real‐life moral beliefs. Arguments do not warrant their conclusions unless the premises of those arguments are themselves warranted, and in the real world, they say, no one would ever be warranted in believing premise (2). Other critics argue that one might conceivably be warranted in believing (2), but only if one were already warranted in believing the conclusion: one could never be warranted in believing that all of Aunt Dahlia’s beliefs were true unless one had checked every one of her beliefs including her belief that Bertie ought to marry Madeline. The obvious reply to this last criticism is that propositions such as (2) may be inductively warranted: one might amass substantial evidence that Aunt Dahlia was infallible without having to check all of her beliefs. In any case, such considerations suggest that the doctrine of the IOF, if it is a fallacy at all, is best understood as a point about epistemology and not about logic.