Hume's Law

Here is Hume's Law which, to use an established formulation, is supposed to say that 'you cannot derive and "ought" from an "is"', embedded in its original context.

 

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 ways 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 shou’d 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 can be a deduction from others, which are entirely different from it [...]. [I] am persuaded, that a small attention wou’d subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceiv’d by reason. (Treatise of Human Nature 1739, III,1,1)

 

Hume's  Law is traditionally used to cast doubts on Moral Realism along lines like these:

 

(HL1) A complete description of the world can be given in terms of 'is' statements.

(HL2) You cannot derive 'ought' statements from 'is' statements.

(HL3) The realisation of moral entities would go along with the truth of some 'ought' statements.

(HLC) A complete description of the world can be given without implying the existence of any moral entities.

 

The idea that there is a difference between 'regular' statements, representing facts, and other statements, representing nothing, or not facts, is initially appealing, and Hume certainly had reason to complain about contemporaries who derived 'ought' statements from 'is' statements rather carelessly. Yet closer inspection reveals that nevertheless, Hume's law is clearly invalid. On a normal, common-or-garden construal of 'is', 'ought' and 'derive' you actually can derive 'ought' from 'is'. 

 

(a) Peter has promised to help Paul. Hence, Peter ought (all things being equal) to help Paul.*

(b) Killing is evil. Hence, you ought (all things being equal) not to kill.

(c) On all accounts, killing is forbidden. Hence, you ought not to kill.

(d) Peter can prevent much suffering by doing x without any risk of drawback. Hence, Peter ought (all things being equal) to do x. 

(e) Peter wants to live, and if Peter wants to live, he should stop taking drugs. Hence, Peter ought (all things being equal) to stop taking drugs.

 

Such examples are usually countered by objections to the effect that the antecedent is not really an 'is' after all (but rather involves an 'ought', though covertly). However, as a matter of fact no conception of the 'is'/'ought' divide has as yet been established which would confirm such contentions, which means that such objections do not seem to be based on the facts (barring terminological conjuring tricks). Now, to be sure, that objections along these lines do not seem to be based on the facts does not strictly mean that they are not based on the facts, and that a construal supporting such objections has not been established does not mean that such a construal cannot be established. Yet notice that unless the relevant construal has been given, and proven to be at least alright plausible, we cannot even say what 'the' objection to Moral Realism is supposed to consist in. And given this, it is clearly quite nonsensical to ascribe any privileged significance to 'the' objection. It is certainly against all proper academic standards to insist on the special significance of an argument whose content is quite unclear.

 

As to the normal interpretation of the 'is'/'ought' opposition: It is part of the Swabian defence to show that 'ought' actually can be derived from 'is' (in the particular sense which actually is relevant for the argument—cf. d in the above list):  If x is able to prevent dis-value, then it follows that he or she ought (all things being equal) to do so—this is an a priori truth; whoever denies it fails to understand the description. 

 

*J.L. Austin seems first to have discovered this particular example; Searle adopts the idea from him in his prominent "How To Derive 'Ought' From 'Is'" (Philosophical Review 73/1 (1964), 43-58).