Or have found that which was lost, and lieth concerning it,.... Who having found anything lost, at once concludes it his own, and converts it to his own use, never inquiring after the proprietor of it, or taking any method to get knowledge of him, and restore it to him; but so far from that, being suspected of finding it, and charged with it denies it: Maimonides gives a reason why a lost thing should be restored, not only because so to do is a virtue in itself praiseworthy, but because it has a reciprocal utility; for if you do not restore another's lost things, neither will your own be restored to you:and sweareth falsely; which is to be understood, not of the last case only, but of all the rest, or of anyone of them, as it follows:in any of all these that a man doeth, sinning therein; by unfaithfulness in a trust, cheating, defrauding, lying, and false swearing.
Moreh Nevochim, par. 3. c.