What philosophical works have been dedicated to the topic of rational decision making, the adoption of values, or how people choose their purposes in life?

A slim, accessible book on part of this question (and only part!) is Decision Theory and Rationality by José Luis Bermúdez (Oxford University Press 2009). It requires little or no technical knowledge of decision theory, and shows how decision theory can't possibly be an exhaustive account or explication of rationality. Bermúdez makes a good case, in simplest terms, that rationality plays at least three key roles: the guidance of action (i.e. answering the question what counts as a rational solution to a decision problem), normative judgement (answering the question whether a decision problem was set up in a way that reflected the situation it is addressing), and explanation (answering the question how rational actors behave and why). He argues that no form of decision theory (there are lots, and he only explores a few of the more common ones) can perform all three of these roles, yet that if rationality has one of these three roles or dimensions, it has to have all three of them. So decision theory can't be the whole story.

But it sounds as if your question goes way beyond decision theory and rationality in the narrow sense. The question how people arrive at their values and purposes or should arrive at them in the first place (decision theory takes for granted that people already have settled and stable values) is not one that philosophers since the ancient world have had much to say about that's of any use. For the first couple of thousand years or so since their time, religion has been the main source of answers to that question. Since the Enlightenment (i.e. for the past quarter millenium or so) it's been explored mostly not by philosophers strictly speaking, but by literary types with philosophical inclinations such as Diderot, Goethe, Dostoevsky, Robert Musil, Marcel Proust, Thomas Mann and so on (not much in English since about George Eliot). Not that religions have given up (including now various brands of secular religion as well), and also there's a huge self-help literature on the subject, much of it religious or quasi-religious, and most of little or no value. If a philosopher (in the modern sense, i.e. an academic philosopher, not in the 18th-century French sense) tries to tell you she has an answer to this question, that is the point where you should stop listening and walk away.

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