Open English Wordnet
Nouns
(n)kernel, substance, core, center, centre, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, pith, sum, nitty-grittythe choicest or most essential or most vital part of some idea or experience“the gist of the prosecutor's argument”, “the heart and soul of the Republican Party”, “the nub of the story”
(n)inwardnesspreoccupation especially with one's attitudes and ethical or ideological values“the sensitiveness of James's characters, their seeming inwardness”, “inwardness is what an Englishman quite simply has, painlessly, as a birthright”
(n)inwardness, internalitypreoccupation with what concerns human inner nature (especially ethical or ideological values)“Socrates' inwardness, integrity, and inquisitiveness” (H.R.Finch)