St. Isaac the Syrian: Purity of mind

Of what does purity of mind consist? The "pure of mind" is not a person who does not know evil things, surely such a one would be an animal. Nor doe we call "pure of mind" those whom nature has placed in a condition of innocence, (cf. Matt 19:12) unless we require that persons do not even belong to the order of created beings. (Job 4:17-18; 15:14-16) But purity of mind is captivation by divine things, which comes about after the practice of many virtues.
+St. Isaac the Syrian, from On Ascetical Life, Third Discourse