Doubts in the mind cannot be removed by correspondence or by studying books; one has to make an effort. One should follow the instructions of the scriptures and the guru; thus one develops faith, and from faith comes purity of heart. Only then are all doubts dispelled from the mind.
(p.214, Spiritual Treasures, Letter of Swami Turiyananda from Varanasi on 19 April, 1920.)