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.

Most readers think of a trilogy as a sequence of books that involve the same characters or setting, but this needn't be the case. For example, the *Sachwörterbuch de Literatur* (page 849) by Gero von Wilpert points out that,

> Seltener und erst in neurer Zeit üblich wird die T[rilogie] in der Epik, bes[onders] im Roman; auch hier sind alle Stufen des Zusammenhangs von der lockeren themat[ischen] Verknüpfung (Typ: Raabes sog[enannte] Stuttgarter Triolgie: *Hungerpastor*, *Abu Telfan*, *Schudderump*) bis zur einheitl[ichen] Durchgestaltung (Typ: Kolbenheyers *Paracelsus*-T[rilogie]) vertreten, (…).

Translation:

> In narrative fiction, especially the novel, the trilogy is rarer and has only become common in recent times; here, too, all levels of connection are represented, from loose thematic linking (type: Raabe's so-called Stuttgart trilogy: *Hungerpastor*, *Abu Telfan*, *Schüdderump*) to unified elaboration (type: Kolbenheyer's *Paracelsus* trilogy), (…).


**References**

* Kerbel, Sorrel (editor): *The Routledge Encyclopedia of Jewish Writers of the Twentieth Century*. Routledge, 2004. (Entry on Koestler by Geoff Sadler.)
* MacAdam, Henry; Cooper, Duncan: *The Gladiators vs. Spartacus, Volume 1*. Cambridge Scholars Publishing, 2020.
* von Wilpert, Gero: *Sachwörterbuch de Literatur*. 8th edition. Stutgart: Kröner, 2001.
* 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.