√IN(ID) root. “mind, (inner) thought, inmost heart, inner senses”
A root appearing in various notes written around 1957 connected to the mind (PE17/145, 155, 189; UT/400; VT43/16). Its most notable derivatives are Q. indo/S. ind “(state of) mind”. In The Etymologies of the 1930s, ᴹQ. indo “heart, mood” and N. ind “inner thought, meaning, heart” were instead derived from the root ᴹ√ID; this 1930s root was unglossed but had derivatives connected to both thoughts and desires, such as ᴹQ. íre “desire, longing” (Ety/ID).
Tolkien introduced a new derivation of indo from √IN(ID) in Quenya Notes written in 1957 (PE17/145, 189):
IN-I-D, mind, inner thought. These refer to the movements or activities of the fëa or “spirit” (rational soul). indo, inner thought, in fea as exhibited in character or [?personality]. indóme, settled character, also used of the “will” of Eru (PE17/189).
The root √IN- “inmost heart, thought, mind” with extended forms √INID and √INIS also appeared in contemporaneous Notes on Names, again as the basis for indo (PE17/155).
Tolkien coined yet another etymology of Q. indo in notes written in 1969, deriving it from √NID “force, press(ure), thrust” (PE22/165). This new derivation doesn’t necessarily invalidate √INID, but rather gives a new basis for the root itself as an vocalic augment of √NID rather than an extended form of √IN-.
References ✧ PE17/145, 155, 189; UT/400; VT43/16