The picture underlying this adjective is that of a piece of cloth or other material, neatly folded once, and without a variety of complicated folds. Hence the idea of simplicity or singleness (compare simplicity from the Latin simplex; semel, once; plicare, to fold). So, in a moral sense, artless, plain, pure. Here sound, as opposed to evil or diseased. Possibly with reference to the double-mindedness and indecision condemned in Mat 6:24.