Written word, too, may be classified as part of it (6)
I believe the answer is:
speech
'part of it' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for speech that I've seen before include "Harangue" , "Talk given to an audience" , "Formal address or discourse" , "'Utterance, talk (6)'" , "Verbal communication" .)