Solving the contract problem is a function of the type of contract model. It is simplistic to imagine that the solution given in civil law to solve the contractual issue is accidental. The contractual model means a pyramidal structure, within its framework, the relationship between contractual principles is defined, and it is clear that the type of relationship between contractual principles, and for example the relationship between the principle of autonomy of will and the principle of contractual fairness, or the relationship between the principle of binding contract and the principle of contractual fairness, has an undeniable effect on how to solve a contractual problem, including the possibility or impossibility of judicial modification of the contract. Even, the meaning and function of the contractual principles are completely related to the position of the principle in the pyramidal structure of the contractual model; the principle of fairness, which is below the binding principle of the contract, cannot have a meaning and function other than the optimal implementation of the contract, and the principle of fairness, which is higher than the binding principle of the contract or higher than the principle of autonomy of will, can be the source of new contractual obligations and the basis for the expansion of the scope of contractual obligations. However, the point is that the contractual model and the type of the contractual model are a function of what is the basis of the validity of the provisions of the contract, and the basis of the validity of the provisions of the contract itself is a function of the nature and type of legal philosophy and, in fact, the type of the basis of the validity of the norm in the legal structure. It is evident that if we face pluralism at the level of validity of the norm, we will also face pluralism at the level of the validity of the provisions of the contract. And, accordingly, the contractual model, and monism in the validity of the norm leads to unity in the validity of the provisions of the contract and the contractual model. Will, reason, and the nature of things can be considered as the three bases that have been competing as the basis for the validity of the provisions of the contract in the context of the modern paradigm. It is certain that the Kantian conception of natural rationality, being the manifestation of Homo noumenon, is more reliable for modernity and this is why it presents the objective will of humans as the basis of the validity of the contract. However, voluntarist positivism, rooted in the thought of Kant as a jurist, and not a moralist, insists on the will of the legislator in place of the objective will of humans as the basis of the contract. The competition between these two foundations was going on throughout the nineteenth century. However, when modernity is faced with a crisis and the solution to the crisis is sought in traditionalism and phenomenology, the nature of things is proposed as the basis of the contract. In fact, the nature of things is a reaction to the extremism of Kant's normativism and the dominance of textualism and legalism based on volitional positivism. Although the original nature of things is the same Aristotelian nature of things. However, jurists who are anxious about the heritage of modernity, i.e. will and right, try to present a conception consistent with modernity, of the nature of things and keep their distance from phenomenologists and traditionalists. Based on this, we can talk about two types of the nature of things, that is, Aristotle's nature of things and the nature of things compatible with modernity. None of these two types of nature of things consider coercion as an inherent element of the contract and are open to contractual dynamics. Aristotle's nature of things requires the contract as a means of realizing exchange justice based on the object and not on the will, and for this purpose, he puts the principle of fairness at the highest; Gény's nature of things recognizes the principle of the autonomy of the will in interaction with social cooperation and the principle of socialization of law. However, Gény's eclecticism finally prevents the judge from considering himself to be in the face of the contractual just and trying to discover it. And he necessarily remains faithful to the principle of social voluntarism. In such a thought, the fourth paragraph of Article1231-5  of the French Civil Code cannot be accepted. Gény's eclecticism is not considered at the international level either. The preference of international documents, including the principles of international commercial contracts, is based on Aristotle's nature of things. Article 3-2-6  of the principles of international commercial contracts, and dominance of the principle of the negation of hardship over the principle of a binding contract, and the acceptance of the judge's authority in amending or terminating the contract, is a good proof of this. The article that seems to have been the model for recruiting article 1195 of the French Civil Code.
The final point is that it is not easy to identify the type of contractual model in Iran's legal system, which is sometimes mentioned as an example of the Romano-Germanic Legal System. On the one hand, with Article 230  of the Civil Code, the Unification Decision No. 805  of the Supreme Court, the lack of formality of the judge's authority to amend the terms of the contract, and considering the predominance of legalism in judicial thinking, which is evidenced by the numerous sentence of courts, regarding the amendment of the obligation and regarding the effect of hardship on the contract, Iran's contractual system can be considered as following the model of the principle of binding contract. And on the other hand, considering the superiority of the principle of LAZARAR over the principle of dominion in Article 132  of the Civil Law, and the domination of the principle of LAZARAR over the principle of binding contract in the opinion of the great jurists, whether by interpreting the negation of the verdict of harm or by interpreting the negation of the verdict in the language of the negation of the subject, and the correctness of the negation of the positivist legalism as the basis of the validity of the civil law norm in Iran's legal system, it seems that the contractual model is close to the model of the principle of fairness; an approach that the author also believes in and will discuss in detail at another time