Is not purity of heart the same as sincerity?

Sincerity, which feigns no good dispositions foreign to the heart, but shows the heart’s truly good dispositions in good deeds, is only the first degree of purity of heart. Purity itself is attained by constant and strict watchfulness over oneself: driving from the heart every unlawful wish and thought and every attachment to earthly things and keeping there without ceasing the remembrance of God and of the Lord Jesus Christ, in faith and love (see the Study: Nous and Spirit, Heart and Soul).