Axiology, also known as theory of the good and value theory (in a narrow sense of that term), is a branch of normative ethics concerned with what kind of things and outcomes are morally good, or intrinsically valuable. This is in contrast to the theory of the right, which is concerned with what people morally ought to do.
The theory of the good is typically aggregative—concerning some quantity which it is better to increase or decrease. However, in some cases people might also include principles (e.g. “an outcome is better the fewer rights violations it contains”).
There are many possible theories of value. Some hold that there is just one source of value, while others rely on multiple sources. Almost all theories agree that wellbeing (also known as “welfare”) has value, and some theories, known as welfarist theories, hold that that it is the only thing which has intrinsic value. Non-welfarist theories recognize other sources of value, such as fairness, equality, or beauty.
Further reading
Greaves, Hilary (2017) Population axiology, Philosophy Compass, vol. 12, pp. 1–15.
Greaves, Hilary & Toby Ord (2017) Moral uncertainty about population axiology, Journal of Ethics and Social Philosophy, vol. 12, pp. 135–167.
Schroeder, Mark (2008) Value theory, Stanford Encyclopedia of Philosophy, February 5 (updated 4 March 2021).
Related entries
consciousness research | hedonism | infinite ethics | intrinsic vs. instrumental value | moral patienthood | moral weight | normative ethics | population ethics | suffering-focused ethics | utilitarianism | valence | welfarism
I think Pablo might in future make an entry on the complexity of value thesis. If and when that happens, that should probably be added to the Related entries here, and vice versa.