Einstein separates two things usually joined together: the work that pays and the work that matters to the person doing it. His advice is to keep science in the second category by earning a living elsewhere, at something one is confident of doing well.

The reason is given in the last sentence. Being paid for research means answering to whoever pays, whether an employer, a funder or an institution, and that accountability changes what questions get asked. Freedom from it is what he treats as the source of joy in the work.

The advice came in a reply to a Californian student in March 1951 who was unsure about a career in astronomy. It reflects Einstein’s own path: the 1905 papers were written while he was employed as a patent examiner in Bern, a post that paid the bills and left his research obligated to no one.