However, consideration must be given as part of entering the contract, not prior as in past consideration. For example, in the early English case of Eastwood v. Kenyon , the guardian of a young girl took out a loan to educate her. After she was married, her husband promised to pay the debt but the loan was determined to be past consideration. The insufficiency of past consideration is related to the preexisting duty rule. In the early English case of Stilk v. Myrick , a captain promised to divide the wages of two deserters among the remaining crew if they agreed to sail home short-handed; however, this promise was found unenforceable as the crew were already contracted to sail the ship. The preexisting duty rule also extends to general legal duties; for example, a promise to refrain from committing a tort or crime is not sufficient. 
Hugo Grotius , the founder of a purely rationalistic system of natural law, argued that law arises from both a social impulse—as Aristotle had indicated—and reason.  Immanuel Kant believed a moral imperative requires laws "be chosen as though they should hold as universal laws of nature".  Jeremy Bentham and his student Austin, following David Hume , believed that this conflated the "is" and what "ought to be" problem. Bentham and Austin argued for law's positivism ; that real law is entirely separate from "morality".  Kant was also criticised by Friedrich Nietzsche , who rejected the principle of equality, and believed that law emanates from the will to power , and cannot be labelled as "moral" or "immoral".