Spock

In a parallel universe, Spock was a member of the crew of the USS Enterprise in the 23rd century.

On one occasion, the Eleventh Doctor recalled memories of his fourth incarnation meeting up with Spock and the command crew of the USS Enterprise to combat a Cyberman invasion of Aprilia III. (IDW: Assimilation²)

Other references
Spock was a character that Rose Tyler associated with a more "professional" approach to the use of technology. Whilst in 1940 London, she once bemoaned the Ninth Doctor's reliance on the sonic screwdriver and conversation as his primary investigative tools. She demanded that he "give [her] some Spock" in their search for a downed Chula ambulance. Later, in an effort to explain to Captain Jack Harkness who the Doctor was, she gave the Time Lord the alias "Mr Spock", because she suddenly realised she didn't even know his proper name. (DW: The Empty Child)

Donna Noble also seemed familiar with the character of Spock. She compared the Tenth Doctor's method of reading a person's thoughts to that of Spock's "mind meld". (NSA: The Nemonite Invasion)

Behind the scenes
The mentions of Mr. Spock in The Empty Child and The Neonite Invasion treat him as a fictional character (as do, implicitly, other references to Star Trek and Star Trek: The Next Generation in Doctor Who stories). Assimilation², of course, treats him as a "real" individual. Reconciling these diegetic levels is an exercise best left to the reader.

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