With increasing pressure of the social interest in the security of transactions through economic development and commercial expansion, the natural-law philosophy slowly affected this simple scheme of formal undertakings legally recognized and enforceable and informal undertakings of only moral efficacy, and brought about the complicated system of enforceable undertakings in the maturity of Roman law with which you are familiar. Four features of this movement are noteworthy. In the first place it led to a juristic theory of formal contract which has affected our ideas ever since. In the strict law the source of obligation was in the form itself. For in primitive thinking forms have an intrinsic efficacy. It has often been pointed out that the faith in legal forms belongs to the same order of thought as faith in forms of incantation and that legal forms are frequently symbols to be classed psychologically with the symbols of magic. The stage of equity and natural law, relying on reason rather than on form, governed by philosophy instead of by naïve faith, looked for the substance and found it in a pact preceding and presupposed by the formal ceremony. Thus a formal contract was a pact with the addition of legal form. The pact was the substance of the transaction. The form was a _causa ciuilis_ or legal reason for enforcing the pact. But if the form was only a legal reason for enforcing something that got its natural efficacy in another way, it followed that there might well be other legal reasons for enforcement besides form. Consequently new categories of contract were added to the old formal contracts and it is significant that while the latter were transactions _stricti iuris_ the former were considered transactions _bonae fidei_ involving liability to what good faith demanded in view of what had been done. In the scope of their obligation these contracts responded exactly to the postulate of civilized society that those with whom we deal will act in good faith and will carry out their undertakings according to the expectations of the community. On the other hand the old formal contracts responded thereto in part only since their obligation was one to do exactly what the terms of the form called for, no more and no less. When one makes _nexum_, said the Twelve Tables, as he says orally so be the law. New categories were added in successive strata, as it were, and juristic science sought afterward to reduce them to system and logical consistency. Thus real contracts, consensual contracts and innominate contracts were added. But it is evident that many of these are juristic rationalizings of what had been done for a long time through formal transactions. Thus the consensual contract of sale with its implied warranties rationalizes transfer by _traditio_ with stipulations for the price and for warranties. The real contract of _depositum_ rationalizes _fiducia cum amico_. The real contract of _mutuum_ rationalizes _pecunia credita_.