Sunesis

Οὐδεὶς γὰρ οὕτως οὔτε μάρτυς ἐστὶ φοβερὸς οὔτε κατήγορος δεινὸς ὡς ἡ σύνεσις ἡ κατοικοῦσ’ ἐν ταῖς ἑκάστων ψυχαῖς.
(Polybius, Hist. 18.43.13)

For no one is such a terrible witness or such a dread accuser as the conscience that dwells in all our hearts. (tr. William Roger Paton)