Kiri-kin-tha was a Vulcan scholar and follower of Surak's teachings.
According to Kiri-kin-tha's First Law of Metaphysics; "Nothing unreal exists."
In 2286, shortly after being brought back from the dead, Spock took a computer memory test while he was on Vulcan and one of the questions the computer asked pertained to Kiri-kin-tha's first law. He answered the question correctly. (Star Trek IV: The Voyage Home; ENT: "The Forge")
This character was only mentioned in dialogue.
The "First Law" appears in the introduction to A Course in Miracles and serves as the foundation for the philosophy laid down in that text.  According to Kirk Thatcher, associate producer of
Star Trek IV: The Voyage Home, Kiri-kin-tha was named after himself.