Fasting
H. W. Beecher. Matthew 17:21 However, this kind goes not out but by prayer and fasting. When the greatest speed of a horse is to be tested, the trainer does not allow him to run at will over in the pasture, nor does he simply put him on a wholesale diet. He almost counts the straws that he gives the horse. He cleans and sifts the oats, and gives him the very best kinds. He measures the horse's exercise, and every part of the horse is under the trainer's watch and care, that he may be in the finest condition when he puts forth his energy in competition. And shall a man do so much for his horse and nothing for himself? Shall there be no preparation, no discipline, no care as to diet, no training, nothing but going on through the linked year, Sabbath joined to Sabbath, taking things as they come, allowing themselves to move about as the current sweeps them along? Is that the wisest method of spiritual culture? (H. W. Beecher.) Parallel Verses KJV: Howbeit this kind goeth not out but by prayer and fasting. WEB: But this kind doesn't go out except by prayer and fasting."