As the world picks through the wreckage of the collapsed crypto exchange FTX and its sister trading firm Alameda Research, scrutiny has fallen largely on founder Sam Bankman-Fried and his close circle of allies. But one hugely important figure in the immense financial catastrophe was never a part of Bankman-Fried’s corporate leadership circle: the 35-year-old Scottish philosopher William MacAskill.

Instead, MacAskill is the thinker perhaps most associated with the moral philosophy known as “effective altruism,” which was profoundly...