On the Continent the second theory, the theory of the inherent moral force of a promise made as such, came to prevail. This was the theory of Grotius. It was generally adopted by Continental writers of the eighteenth century and, as has been seen, it broke down the Roman categories and led to the rule that a promise as such, intending a legal transaction, created legal obligation. At the end of the eighteenth century Lord Mansfield came very near establishing it in our law by his doctrine that no promise made as a business transaction could be _nudum pactum_. But he was too late. Growth stopped for a season and the nineteenth century set itself to systematize and harmonize what it had received rather than to carry the development further. When the natural-law foundation of enforcing promises crumbled, the metaphysical jurists sought to provide a new one. Kant said that it was impossible to prove that one ought to keep his promise, considered merely as a promise, and deduced contract from property as a form of conveyance or alienation of one's substance involved in the very idea of individual rights. So far as consistent with abstract freedom of will according to a universal law one might alienate his services as well as his property, and an undertaking to perform something was an alienation of that sort. This view was generally taken so that while the seventeenth century sought to rest rights upon contract and the eighteenth century rested contract on the inherent moral significance of a promise, the nineteenth century, making the philosophy of property the important thing, rested contract on property. Three of these theories are worth a moment's notice.