See Prolegg. ch. v. §3 b. The letter may have been finished (see §§3, 11) after Ath. had returned home, but the language of §1 seems to be applicable only to his residence at Treveri, and §11 may be reconciled to this supposition. In this case (§1 sub. fin.) it was probably begun as early as the Easter of 337; cf. Letters 17 and 18.