The Aristotle, named after the Earth scholar, was a command ship in the Combined Galactic Resistance which fought against the Daleks. It was originally a hospital ship, but was repurposed because, according to its commander Morgan Blue, "Daleks [didn't] leave any wounded and [the Resistance] didn't take any prisoners".
Despite Morgan's words, the Aristotle did hold one prisoner: "Rusty", the first allegedly moral Dalek. When the Twelfth Doctor arrived on the ship, he, Clara Oswald, Journey Blue, Ross and Gretchen Carlisle were miniaturised in order to give them access to the inside of Rusty's casing in order to repair it.
However, once Rusty was repaired, it returned to its former immoral state and began a rampage inside the Aristotle. It also sent a distress call to a nearby Dalek flying saucer, and a battle ensued aboard the ship between the Daleks and the resistance fighters. The battle only ended once the Doctor telepathically linked with Rusty, returning its hatred towards the Daleks, and Rusty ordered the Dalek saucer to retreat and killed the ones already onboard. (TV: Into the Dalek)
Behind the scenes Edit
As it appears in Into the Dalek, the Aristotle was scripted to appear "old and battle-scarred".