The three novels are obviously not connected at the level of the plot or the story. Instead, the novels are linked by a common theme, which has been summarised as "the nature and ethics of revolution" (Kerbel, p. 546). For this reason, they have also been referred to as the **"ethical trilogy"** (see e.g. Ward), a term that Koestler introduced in his autobiographical work *The Invisible Writing*.

In *The Routledge Encyclopedia of Jewish Writers of the Twentieth Century* (edited b Kerbel) Geoff Sadler writes,

> In these novels Koestler examines the early idealism that gives rise to revolutions, and contrasts it with the later corruption and subversion of those ideals.

In *Stranger to the Square* (published posthumously in 1984) Koestler himself also commented on this (quoted in *The Gladiators vs. Spartacus, Volume 1* by Henry MacAdam & Duncan Cooper, 2020, page 348):

> *Arrival and Departure* was the third volume of a trilogy of which the central theme is the conflict between morality and expediency—whether, or to what extent, a noble end justified ignoble means. It is a hoary problem which obsessed me during the years spent as a member of the Communist Party.

**References**

* Kerbel, Sorrel (editor): *The Routledge Encyclopedia of Jewish Writers of the Twentieth Century*. Routledge, 2004. (Entry on Koestler by Geoff Sadler.)
* Ward, Michael J.: [*The development of spirituality and ethics in the work of Arthur Koestler, 1937-1959*](https://era.ed.ac.uk/handle/1842/30890). Doctoral dissertation, University of Edinburgh, 1997. 
* MacAdam, Henry; Cooper, Duncan: *The Gladiators vs. Spartacus, Volume 1*. Cambridge Scholars Publishing, 2020.