If we assume metaphysical naturalism, then nothing exists outside the world of 'is' (i.e. that which is open to empirical investigation). This implies that either normative concepts (such as 'ought') can be reduced to a purely descriptive form, or else those concepts do not refer to anything real.
Now this poses something of a problem for us, since the phenomenology of our experience suggests to us that normative facts (eg "you should not lie") are of an entirely different nature than descriptive facts (eg "snow is white"). However, Alonzo Fyfe suggested that this is simply because "ought" is a particular subset of "is", with noticable distinctions which set it apart from the typical subsets. Rather like whale is a type of mammal, despite being of an entirely different nature from, say, dogs.
So what type of real-world entity does "ought" refer to? From where does it get its apparent prescriptivity? What makes it different from normal descriptive facts like 'snow is white'? The easiest way to approach these questions is probably to forget about morality for a moment, and just consider "prudential oughts", or hypothetical imperatives.
Consider the following examples:
- John wants to watch The Simpsons, so what channel should ('ought') he set his TV to?
- You should leave now if you want to get to the movie on time.
- I've got a big exam tomorrow, so I really should study.
In effect, "If you want Y, then you ought to do X", is just another way of saying "Doing X is such as to fulfill the desires in question" (where the desires in question are 'Y'). They mean the exact same thing (assuming a truth-conditional theory of meaning). Yet the second sentence is purely descriptive - it makes no mention of 'ought', it is purely within the world of 'is'. Thus we have successfully bridged the is/ought gap, at least for prudential 'oughts'.
Now we are in a position to answer where prescriptivity comes from - in a world of 'is', how is 'ought' even possible? The answer is found in human desires. The proposition "You ought to X" has no prescriptive properties in and of itself (the words are purely descriptive, as the translation in the previous paragraph demonstrates). Any prescriptivity is attached to the proposition by us, and the source of the prescriptivity is our desire-set.
The most widely accepted theory of human action is BDI theory, or Belief-Desire psychology. The core of this theory states that people always act so as to fulfill their (present) desires, according to their beliefs (about how to best achieve this). So if we gain a new belief about how to best fulfill our (present) desires, then we will (as a brute fact of human nature) act on that belief. To pre-empt the obvious counterexamples: If I continue to smoke despite knowing that it is bad for my health, that simply shows that my current desire to smoke is stronger than my current desire for future health benefits.
The end of all human action is the fulfillment of our desires. We use the word/concept 'ought' to bring attention to possible courses of action which (it is believed) will help attain that end. Desires are real, actions are real, and the relationship between them (i.e. "ought") is real. It is thus intrinsically descriptive in nature - the prescription gets attached only when human nature is added into the equation. Our desire set is precisely the list of prescriptions which govern our actions.
You might wonder if this makes the notion of 'ought' redundant: after all, if we always act to fulfill our desires anyway, what do we need this extra notion of 'ought' for? But note that we only act according to our current desires, whereas the concept of 'ought' can refer to the fulfilment of any desire (or set of desires). Thus in the above example of the smoker, we can sensibly say that although I might in fact smoke (because it fulfills my present desires), I 'ought' not to (with regard to the fulfilment of my future desires).
So prudential 'oughts' are part of the world of 'is' (the real world), and can be described purely in terms of real-word entities - such as desires and actions. Specifically, I claim that "You (prudentially) ought to do X", is truth-functionally equivalent to "Doing X is such as to fulfill the desires in question".
But what about morality? The moral 'ought' is somewhat different, since it is considered to (somehow) be binding on agents, regardless of their personal desires. The moral objectivist posits a notion of 'ought' which is supposedly independent of desires. Yet according to the above analysis, offering an 'ought' without a corresponding desire set (for the ought to be 'in regard to'), seems to be as incoherent as offering a location without a corresponding reference location (eg "the keys are above the table" makes sense, whereas "the keys are above" does not!).
This problem will be further examined in future posts...
[Update: this post is seriously flawed -- see my more recent posts on 'Empty Definitions' and 'Is Normativity Just Semantics?']