Cthia was a Vulcan word, typically translated into English as "logic", but actually possessing the more complex meaning of "reality-truth". The term cthia was also often used to refer to the philosophy/religion of logic, emotional control, and pacifism preached by Surak. (TOS novel: Spock's World)

Cthia underwent many phases in its history. In the 22nd century, the variation of cthia practiced by many Vulcans involved considerably less emotional discipline than was often found in later practitioners. This was a result of the numerous interpretations of inaccurate recordings of Surak's teachings. One group's particular take on cthia was particularly unpopular -- the Syrannites -- until the discovery of the Kir'Shara in 2154 by United Earth Starfleet Captain Jonathan Archer. With the discovery of the Kir'Shara, Surak's original set of teachings, cthia was revolutionized, and the tenants of Syrannitism came to dominate modern cthia. (ENT episodes: "The Forge", "Awakening", "Kir'Shara")

Cthia was harder to sustain for those who were not wholly Vulcan. (TOS novelization: Star Trek)