Why did St. Aphrahat the Persian sage focus on the internal prayer of the mind?

It is called the prayer of the mind, prayer of the heart, silent prayer, or hidden prayer, or sometimes, contemplative prayer. The fourth demonstration of St. Aphrahat stands out as one of the oldest extant Christian texts on prayer. It’s discussion of the Lord’s Prayer differs from other well-known writings on the subject of prayer from this period, such as those by Tertullian, Origen, St. Gregory of Nyssa, St. Cyprian, St. Augustine, and others. It is characterized by its wealth of examples from scripture that emphasize that purity of heart is essential as a prerequisite for a prayer to be accepted. St. Aphrahat considers prayer to be the inner encounter of the pure heart with the holy God; prayer is the dialogue of the heart which cannot be pleasing to the holy God unless it is pure and clean.

St. Aphrahat introduces this Demonstration thus: “Purity of heart is a prayer, more so than all the prayers which are prayed in a loud voice, and silence being joined together with a luminous mind is better than the loud voice when a person cries out. My beloved, now give me your heart (Prov. 23:26) and your mind, and hear about the power of pure prayer and see how our righteous forefathers were triumphant before God through their prayer and how it became a pure offering (Mal. 1:11) for them.”²⁴⁸