Philosophical progress
Philosophical progress seems like (1) a cause area in itself; (2) relevant to multiple specific other cause areas; and (3) a part of cause prioritization.
Answers to philosophy questions have implications for altruism (see e.g. the work of MIRI and Foundational Research Institute) and for specific cause areas (like AI alignment).
For cause prioritization, answers to questions like how we discover our values or which beings count for moral consideration are relevant to deciding which causes to support. See e.g. this post and this report. In this sense topics like population ethics are a subset of making general philosophical progress.
See some discussion about philosophical ability here. See also this Quora question.
See also this comment by Wei Dai.
Paul Christiano weighs in here.
