Karl Heinrich Marx was a figure central to communism who wrote the major work Das Kapital. (PROSE: History 101; AUDIO: Brotherhood of the Daleks) The Doctor met Marx prior to or during his sixth incarnation. (AUDIO: Brotherhood of the Daleks) He had once lent his reader's ticket for the British Museum to Marx. (PROSE: Timewyrm: Apocalypse)
Behind the scenes
In The Sun Makers, Mandrel and the Fourth Doctor have the exchange, "What have we got to lose?" "Only your claims". This riffs on a common paraphrase of the final lines of The Communist Manifesto by Marx and Friedrich Engels, "What have you got to lose? Only your chains." The official English translation of the line is "The proletarians have nothing to lose but their chains."