Idea

An idea was a simple, immaterial concept.

History
In 43K2.1, the Tenth Doctor once pointed out that "an idea [was] hard to kill", when realising that the Beast had escaped its prison despite its physical form being trapped by turning itself into the "idea" of the Devil. (TV: )

Undated events
Alydon claimed that, prior to the Neutronic war, Skaro was "full of ideas". (TV: )

Early on in his life, the Fifteenth Doctor reflected on how the "pantheon of enemies" he'd face had "no idea what they'd let themselves in for." (PROSE: )