John Cassian, Conferences I–X VI.12: How the upright man ought to be like a stamp not of wax but of hard steel

And so the mind of the upright man ought not to be like wax or any other soft material which always yields to the shape of what presses on it, and is stamped with its form and impress and keeps it until it takes another shape by having another seal stamped upon it; and so it results that it never retains its own form but is turned and twisted about to correspond to whatever is pressed upon it. But he should rather be like some stamp of hard steel, that the mind may always keep its proper form and shape inviolate, and may stamp and imprint on everything which occurs to it the marks of its own condition, while upon it itself nothing that happens can leave any mark.