1950, coined by U.S. writer L. Ron Hubbard (1911-1986). In "Dianetics: The Modern Science of Mental Health" (1950), the glossary connects the word to Greek dianoua "thought." "Self Analysis" by Hubbard (1951) quotes an etymology said to have been added to the Funk & Wagnalls New Standard Dictionary: "Gr. dianoetikos--dia, through plus noos, mind."
There was an earlier dianoetic (1670s) "of or pertaining to thought," from Greek dianoetikos "of or for thinking; intellectual," from dianoetos, verbal adjective from dianoe-esthai "to think," from dia- "through" (see dia-) + noe-ein "to think, suppose," from noos "mind," which is of uncertain origin.