An enemy and an envious person is an eye over his neighbour, watching narrowly into his conduct; but if known to be so, he may be highly useful to him by putting him on his guard: knowing he is watched by one who is disposed to put the worst construction upon his actions, he will be so cautious, as to give him as little opportunity as possible of doing him an injury: he, therefore, may be said also to afford an additional eye to his neighbour; which is the more direct meaning of the adage.