the mind > mental capacity > knowledge > hiding, concealing from view > privacy > [adverb] > to oneself (3)
In heart; in mind or thought; in spirit. (Hence implying ‘in reality, sincerely, at heart’, or ‘secretly’.)
in secret: in private, not openly or in public; secretly. †at one's secret: to oneself, privately. †of secret: of a secret character.