Ultra posse nemo obligatur

Ultra posse nemo obligatur is a Latin legal term, meaning, "No one is obligated beyond what he is able to do."

Ultra posse nemo obligatur has its origin in the Roman law. The expression can be found in Justinian's Digesto.

A common variant of the phrase ultra posse nemo obligatur is ad impossibilia nemo tenetur.[1][2]

As said it has just been mentioned, in legal thinking a very well-known maxim was originally coined by the Roman jurist Celsus expresses this general idea impossibilium nulla obligatio est (Digest 50, 17, 185).45 Similar versions of the same maxim are, for instance, nemo potest ad impossibile obligari,46 ultra posse nulla obligatio o nemo obligatur, and ad impossibilia nemo tenetur. This legal maxim is usually seen as a corollary of the philosophical moral principle ‘ought implies can’.47 It is worth noting that in law the aforesaid implication is interpreted in the sense of Charles I. Lewis’ strict implication,48 as a presupposition, rather than as it often happens in deontic logic as the so-called material implication. In other words, in the eyes of jurists, ‘ought implies can’ means that it is necessary that what it is prescribed is possible in order to be obliged. Where, of course, the point is in which sense we speak of possibility in this regard.[1]