What are unjust laws?
It can be said that unjust laws are contrary to Justice, that is, contrary to the greater objective of Law which, as already said, is to give each one what belongs to him.
By some distortion, the norm becomes useless in the function for which it is intended, and an injustice occurs. It may be that this distortion occurs in three ways: by destination, when the legislator’s intention is exactly to create an unjust norm, causal, which occur due to political incompetence, which produces laws that deviate from their purpose and eventual , when, in the face of the concrete case, the norm may become unfair.
There is a discussion among jurists about the validity or not of these unjust laws. Some think that the norm cannot be valid because it contradicts the ideal of Law; while others point to validity, for fear of causing an even greater harm, which would be the non-observance of the Law in certain situations, under the pretext of injustice. Thus, the most plausible would be to comply with the provisions of the law, but doing an interpretation work to avoid the evil contained in the norm.
Unjust laws and the obligation to obey the law according to John Finnis
Before starting our explanation of this specific point of Finnis’ Theory of Natural Law, which corresponds to unjust laws, we must here make a brief introduction to the study of John Finnis in order to better understand the objective of this work, which corresponds to understanding how Natural Law de Finnis talks about unjust laws.
Finnis’ starting point in the development of his Theory is to demonstrate that natural law does not derive from empirical factors but from “principles of practical reason”.  Such arguments of the author aim to defend the point of view of Saint Thomas Aquinas who, unlike what other authors spoke, never stagnated in naturalistic fallacy, as his basement on good and bad, for example , do not incur on empirical factors but on “first principles”. In other words, such factors are not based on deductions but on “self-evidence”, since it is enough to reach them through reason.
However, according to Finnis, there is such misunderstanding about the criteria for the realization of Natural Law, the same misunderstood and repudiated by several authors of the time such as Kelsen, Hart and Raz  . And, to solve this problem, Finnis proposes in his book Natural Law and Natural Rights some forms of analysis by the theorist so that he goes beyond the theoretical description and without involving in the evaluative task what would lead him to practical reasonableness itself.
In other words, according to Finnis, one must observe the relevance of starting from the internal point of view, since the theorist cannot be “seeing everything from the outside”, he must be an evaluative observer who can give his opinion on decisions. Based on this, according to John Finnis, one must choose the social attitude that converts the Law as an institution that lacks respect and obedience, so that happiness and social peace can be promoted. Such thinking results in what Finnis has widely discussed is so-called “practical reasonableness”. According to the author:
The main concern of a theory of natural law is to explore the requirements of practical reasonableness in relation to the good of human beings who, by living in community with one another, are confronted with problems of justice and rights, of authority, law and obligation. (FINNIS, 2007, p.337).
Having made these explanations, we can point out that Finnis intends, with his reasoning on reasonableness, to warn that the law goes beyond the law and that it is based on values. These values, which can be explained objectively, reaching through the analysis of the very actions and institutions that men create. Thus, it is up to the Theory of Natural Law to study such values.
Therefore, according to Finnis, reasonable demands correspond to those that promote happiness and social peace (what the author will call the “common good”), since this is what will determine what a “just law” is.