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The practicality objection
The 'practicality objection' (following Feldman) is that :
in many cases we cannot determine which of our alternatives in fact would lead to the best outcome (Feldman: 49). In other words, the objection is that in many cases, classic utilitarianism ('an act is morally right iff it leads to the best outcome' - Feldman: 49) cannot provide a decision-procedure, as John Rawls called it, for ethics. A normative ethical theory such as classic utilitarianism should be action-guiding; but classic utilitarianism isn't so, because it leaves the best outcome indeterminate. (Fred Feldman, 'Actual Utility, the Objection from Impracticality, and the Move to Expected Utility', Philosophical Studies: An International Journal for Philosophy in the Analytic Tradition, Vol. 129, No. 1, Selected Papers from the 2004 Bellingham Conference (May, 2006), pp. 49-79: 49.)
In the following extract John Broome states and considers one objection - which isn't to say there are not others - to turning to expected utility utilitarianism to overcome the impracticality objection.
The basic point is that expected utility utilitarianism provides no more of a decision procedure than does the standard utilitarianism.
Expected utility utilitarianism does not overcome the objection
Suppose there are h people. Each has preferences among a set of
alternative prospects, the same set for everyone. Each person's preferences satisfy the axioms of expected-utility theory-I shall call
such preferences coherent. Expected-utility theory tells us that coherent preferences can be represented by a utility function. This
function assigns a utility to each prospect in such a way that, of any
two prospects, the preferred one has the higher utility. The function
will also be expectational, by which I mean that the utility it assigns
to a prospect whose results are uncertain is the mathematical expectation of the utility it assigns to the results. If a person's preferences
are coherent, there are actually many expectational utility functions
that will represent them, all positive linear transforms of each other.'
Suppose there are also social preferences among the same set of
prospects. If these too are coherent, they can be represented by an
expectational utility function. Once again there are actually many
expectational utility functions that will represent them, all positive
linear transforms of each other. (John Broome, 'Utilitarianism and Expected Utility', The Journal of Philosophy, Vol. 84, No. 8 (Aug., 1987), pp. 405-422: 405-6.)
At both the individual and the social level, there will not be a unique utility function but actually many expectational utility functions; a unique course of action, derived under the conditions Broome describes, is not to be had.
Reply to save expected utility
But isn't the right response that we can be indifferent between these utility functions since all offer equally good prospects ?
Counter-reply to sink it
The conditions which expected utility utilitarianism assumes are in fact unrealistic, and therefore the theory is impractical to rely on. What are the chances that 'Each person's preferences satisfy the axioms of expected-utility theory'? That they are coherent and can be represented by a utility function 'in such a way that, of any two prospects, the preferred one has the higher utility'?
Even if the assumed conditions do apply, how realistic is it that we can collate the necessary knowledge of utility functions at the time of decision-making? And more than that, do so for every decision among the indefinitely many decisions we have to make?Very unrealistic, I suggest.