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, (…).
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), (…).
- 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. Doctoral dissertation, University of Edinburgh, 1997.