True wisdom is gazing at God. Gazing at God is silence of the thoughts. Stillness of mind is tranquility which comes from discernment.
-Saint Isaac the Syrian
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Discernment in beginners is true knowledge of themselves; in intermediate souls, it is a spiritual sense that faultlessly distinguishes what is truly good from what is of nature and opposed to it; and in the perfect, it is the knowledge which they have within by Divine illumination, and which can enlighten with its lamp what is dark in others. Or perhaps, generally speaking, discernment is, and is recognized as, the certain understanding of the Divine will on all occasions, in every place and in all matters; and it is only found in those who are pure in heart, and in body and in mouth.
-St. John Climacus, "The Ladder of Divine Ascent"
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So, then, the four kinds of discernment to which I have been referring will be necessary to us. First, as to material, is it true gold or spurious? Second, we must reject as fake and counterfeit coinage those thoughts which have the deceptive appearance of piety. They bear a false and not the genuine image of the king. Then we must be able to detect and to abhor those which impose a viciously heretical stamp on the precious gold of Scripture. This is not the effigy of the true king but of a tyrant. Finally, we must drive away thoughts which are like underweight coins, dangerous and inadequate, thoughts which have lost weight and value because of the rust of vanity, thoughts which do not measure up to the standard of the ancients.
-St. John Cassian, The Conferences