The following quote comes from Darwall, S. 2001. "Because I want it." Social Philosophy and Policy 18: 129–153.
What we should say is that the imperatives or prescriptions that follow from instrumental reasoning are imperatives of practical consistency. They tell the agent either to take the means or to give up the end or the belief that the means in question is the only means. They do not tell the agent, If A is your end, and B the only means, then you should do B.
Dave Phillips quoted it in his defense of the thesis that J.L Mackie provides a better theory of practical reason than Bernard Williams in "Mackie on Practical Reason" in Philosophical Studies Series, A World Without Values , Essays on John Mackie's Moral Error Theory, (Richard Joyce and Simon Kirchin (eds.)).
One of the alleged advantages is that Williams gives reasons generated by desires too much authority. Darwall's account, which matches Mackie's in this regard makes reasons grounded on desires provisional, as described above.
This does not strike me as being at all practical.
"Either pull your hand out of the fire or give up the end of avoiding pain and charred flesh or the belief that pulling your hand out of the fire is the only way to avoid pain and charged flesh."
The recommendation treats desires as if there is a mental switch somewhere that simply turns the desire off. As anybody with an addiction or some other bad desire, as anybody mourning the loss of someone they truly and deeply loved such as a parent has for their child, as anybody with a fear or a love that they wish not to have will testify, there is no such switch.
Altering ends takes work.
The accurate account is, in fact, "Given that A (avoiding severe pain) is an end, and B is the only means, then you should do B." Or, at least, "you have a reason to do B". The agent certainly has a reason to pull his hand out of the fire, even if there are other ends providing more and stronger reasons to endure the pain. Sometimes "should" refers to all of the reasons an agent has; sometimes to a subset. In the statement above, "should" refers only to the aversions to pain and charred flesh.
Even if it were possible to switch off one's desires/ends, this suggestion still has a significant problem - as least as it is presented.
The agent has been given a choice among three options; take the means, give up the end, or find another means.
How are we supposed to make this choice? Where do we find the reasons that we can apply to picking one of the three? Whatever reason we find, it is going to come with its own three options; to accept those means, reject the ends, or find a new means. This choice, in turn will come from reasons having the same three options to chose from.
We do not seem to be making any progress here.
I am going to stick with Bernard Williams' account of practical reasons for now - with one amendment.
A has a reason to φ if and only if A has some desire the satisfaction of which will be served by his φing.