I hear it asserted that David Hume said one cannot deduce an "ought" from an "is". I also find it asserted that he said reason must be only the servant of the passions.

I had long uncritically thought that the argument was: one cannot deduce an "ought" from an "is"; THEREFORE reason can only serve the passions.

I've never thought much about these questions, but some views of C. S. Lewis in his book The Abolition of Man have raised a question in my mind: Is it generally held that if one cannot deduce "ought" from "is", then reason can only serve the passions? Or are there philosophers who hold that one cannot deduce "ought" from "is", but reason is not and ought not to be only the servant of the passions?

PS: I put a "reason" tag on this thing and the software changed it to "logic". Surely those are not synonymous? Shouldn't there be a "reason" tag?

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    Regarding your PS: What definition of reason are you looking for that logic doesn't cover?
    – stoicfury
    Commented Dec 30, 2011 at 7:07
  • @stoicfury : I'm not sure where the boundaries of "logic" are, but I'd think maybe concept-formation and the norms by which one might decide what are good concepts might be included within "reason" but not necessarily within "logic". Commented Jan 1, 2012 at 20:05
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    @stoicfury: I think a case can be made for reason to include logic, but not the other way around. Perhaps that could be a question for the site? ;-) Commented Feb 2, 2012 at 0:31
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    Abolition of Man is excellent and one of my favorite essays. Which particular views prompted the question, by the way? Commented Feb 2, 2012 at 0:40
  • @JonEricson : Lewis maintains that basic moral principles are based on reason, but that they are not deduced; he says Kant was right to say that the imperative is categorical. Commented Jul 25, 2012 at 3:34

3 Answers 3


Plato certainly does not believe that the reason should be a servant of the passions; in The Republic, he makes his belief quite clear that in fact, reason (one of the three parts of the soul, according to him) should make passion its servant and control it with rationality. Therefore, Plato satisfies the latter part of your question.

Now, for the former. Plato's legendary allegory of the cave is the epitome of what is: a society of imprisoned and prejudiced people. From this allegory, he explicitly describes what ought to be as the escape from the cave; something that rarely (if ever, in reality) happens. Clearly, Plato does not believe that one can deduce an "ought" from an "is," but rather the opposite: he often uses what "is" from the world as an example of what "ought not be."

Other examples of this are when he attacks common views of justice (what "is") and shows that they are unsatisfactory ("ought not be"). In Meno, he similarly attacks common views of virtue and shows that they too are unsatisfactory. Although these are examples of beliefs that cannot be turned into "oughts," and you are talking about facts that cannot be turned into "oughts," the two are parallel in Plato's (often multilayered) writings.

This is just one philosopher who I think satisfies what you're looking for; I'm sure that there are more I do not know of.


I think in order to contradict Humes statement, that reason can only serve the passions, it is necessary to disprove his ought-is law, because there are only two kinds of ought: the ethical and the functional.

The philosopher David Johnson tried to solve the is-ought problem with a logical workaround in his book "Truth without paradox". His argument works like this:

  1. The sky is blue or adultery is wrong.
  2. It is not the case that the sky is blue.
  3. Therefore adultery is wrong.

(Although the second premise is not true, this doesn't invalidate the argument, it only isn't sound.)

But as I said it is just a workaround and not really convincing.


'The passions' are only one 'source of ought' (to say: 'motivation')- certainly if we wish to achieve something (presumably at the behest of the passions) then there are more efficient ways of doing so than others- reason would then certainly say (in obeisance to his masters the passions) that one ought to plump for ways among the most efficient.

But there is, at least theoretically, more than one way to skin a deontological cat, G.E. Moore, for example, actually used an argument expanding on the impossibility of deriving an "ought" from an "is" to conclude that morality must be another innate driving force separate from both reason and the passions.

My personal favourite approach is that of Hare, who argued that 'ought' is really just part of a very human language game, in which we make 'universal prescriptions' about people's behaviour.

For a thoroughly accessible introduction to these and other (potential, arguable) circumnavigations of Hume's guillotine- check out Peter Singer's Introduction to Ethics essay.

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