User blog comment:Crazy sam10/Some thoughts surrounding Ghosts/@comment-24.13.48.199-20140312000922/@comment-1055346-20140322035040

For Diego and Reference, the specific quote is taken from a Kotaku articl:

"Game Director Dave Anthony hinted that we may find out more about "imaginary" Viktor Reznov. "He was essentially a figment of the player's imagination," Anthony said.

"Or was he?" Studio Director Mark Lamia chimed in, playfully. "Will we find out more about that?" asked Anthony with a smirk."

http://kotaku.com/5906808/48-things-that-you-should-know-about-call-of-duty-black-ops-ii