The distinction between the λόγος ἐνδιάθετος, thought, and the λογος πορφορικός, speech, appears first in Philo. II. 154.  On the use of the term in Catholic Theology cf. Dr. Robertson’s note on Ath., De Syn. § xxvi. p. 463 of the Ed. in this series.  Also, Dorner, Div. I. i. p. 338, note.