"Pure concentration is like a two edged sword; it has the edge of single pointedness or stability, and it has the edge of clarity and intensity. To attain single pointedness at the beginning we have to make great effort, but if we apply too much effort we lose our single pointedness and our mind begins to waver towards another object. We need to adjust our effort so it is not too tight and not too loose, rather like a musician has to adjust the strings of an instrument."
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