Sebastian, or Ruling Passions (1983), is the fourth volume in The Avignon Quintet series by British author Lawrence Durrell, which was published from 1974 to 1985.

It continues the story of Constance and a Gnostic cult, which was introduced in the first novel of the quintet, Monsieur (1974).

A letter informs the Egyptian Sebastian Affad that he will die; a mix up has caused major ructions within the Gnostic sect in Egypt.

Critic Graham Hough of the London Review of Books wrote about the need to read all the novels in this series.

But he also wrote, "As it happened, I read them in the wrong order, but such is the vitality and attack of Durrell’s writing that it hardly mattered.