From the treatise On Spiritual Perfection by Diadochus of Photice

“Therefore, we must maintain great stillness of mind when in the midst of our struggles.  We shall then be able to distinguish between the different types of thoughts that come to us: those that are good, those sent by God, we will treasure in our memory; those that are evil and inspires by the devil we will reject.  A comparison with the sea may help us.  A tranquil sea allows the fisherman to gaze right to its depths.  No fish can hide there and escape his sight.  The stormy sea, however, becomes murky when it is agitated by the winds.  The very depths that it revealed in its placidness, the sea now hides.  The skills of the fisherman are useless.”