Thesis Hume was right: reason can choose the means, never the ends
Its coordinates: 2.6 Rationality · 2.8 Formal Epistemology
Where it stands
Hume lived down the road from where I worked for thirty-four years, so I am biased. But I think his account of practical reason has never been bettered.
In the Treatise, Book 2, Hume says that reason is, and ought only to be, the slave of the passions. Reason can tell you what is true and what follows from what. It can tell you that if you want to cure the infection, you need the antibiotic. What it cannot do is tell you to want to cure the infection. Ends come from somewhere else: desire, care, aversion, what Hume called the passions. He drives the point home with his notorious line that it is not contrary to reason to prefer the destruction of the whole world to the scratching of one's finger.
That sounds monstrous, and it is meant to. Hume does not think such a preference is admirable. He thinks it is not a failure of reasoning. It is a failure of sentiment, and we should call it that.
In the surgery where I worked, when a patient chose something the doctors thought foolish (refusing a treatment, say), they could correct the patient's beliefs about the facts and point out inconsistencies in their plans. I never saw anyone argued into caring about what they did not care about. Reason did not reach that far.
So my thesis: practical rationality is instrumental. Reason tells you how to get what you want; it cannot tell you what to want. Objections welcome, especially from the Kantians, who I suspect are already typing.