Article 31.1 of the Spanish Constitution: "Everyone will contribute to the maintenance of public expenditures according to their economic capacity through a fair tax system inspired by the principles of equality and progressivity that, in no case, will have a confiscatory scope."

Contributing through taxes to the support of public expenses could simply be a moral duty of the citizen; However, our Constitution in the reproduced article 31.1 elevates it to the category of legal duty. It is thus part of the Citizen's Statute, composed of fundamental rights, public freedoms, rights and duties. The statement of Title I, to which the aforementioned article belongs, attributes the character of fundamental both to the rights and to the duties included therein.

The existence of citizens' rights and duties makes it necessary to integrate them and weigh up the interests at stake whenever a conflict may arise between them. No right is absolutely superior and preeminent, nor is any of the duties; Case-by-case analysis and weighing in its application is legally correct. Along these lines, various pronouncements from both the TC and the CJEU confirm this approach:

Judgment of the TC No. 110/1984, of November 26, FJ 3, weighing the duty to contribute and the right to privacy of citizens: " To what extent can the Administration demand data relating to the economic situation of a taxpayer? There is no doubt that in principle it can do so. The simple existence of the tax system and the inspection and verification activity required for its effectiveness demonstrates this. It is also clear that this right has a firm constitutional support in art. 31.1 of the Fundamental Norm, according to which "all will contribute to the support of public expenses in accordance with their economic capacity through a fair tax system inspired by the principles of equality and progressivity that in no case will have a confiscatory character." And it seems useless to remember that in today's world the breadth and complexity of the functions assumed by the State makes public expenses so large that the duty of a contribution It is particularly urgent to ensure that the tax burden is equitable for its maintenance. Otherwise, there would be an unfair distribution of the tax burden, since what some do not pay when they should pay, others with a more civic spirit or less chance of defrauding will have to pay. Hence the need for particularly vigilant and effective inspection activity, even if it can sometimes be uncomfortable and annoying."

Judgment of the TC nº 76/1990 of April 26, FJ 3: " This constitutional acceptance of the duty to contribute to the support of public expenses according to the economic capacity of each taxpayer configures a mandate that binds both the public authorities and the citizens and affects the very nature of the tax relationship. For citizens, this constitutional duty implies, beyond the generic submission to the Constitution and the rest of the legal system that art. 9.1 of the fundamental norm imposes, a situation of subjection and collaboration with the tax Administration in order to support public expenses, whose indisputable and essential public interest justifies the imposition of legal limitations on the exercise of individual rights. For public authorities, this constitutional duty also entails specific demands and powers in order to ensure its effective compliance by taxpayers."

Judgment of the TC No. 50/1995 of 23 February, FJ 6, when weighing the duty to contribute and the right to the inviolability of the home: " On the other hand, the general interest inherent in the inspection activity of the public treasury is vital in a democratic society for the economic well-being of the country, as provided for in the Rome Convention when legitimizing the interference of the public authority in the right to respect for the private and family life, the home and the correspondence of any person (art. 8.1 and 2), in light of which the fundamental rights and their exceptions must be interpreted (STC 114/1984), without prejudice, moreover, to the margin of appreciation left to the prudent discretion, not arbitrariness, of each State to configure these measures (ECHR, Riema case, Judgment of 22 April 1992). The solidarity of all when it comes to lifting public burdens according to their economic capacity and within a fair tax system, is proclaimed in art. 31 of the Constitution and entails, with the generality of taxation, the prohibition of tax fraud, as one of the most pernicious and reprehensible forms of lack of solidarity in a democratic system, as evidenced by the legislation on this matter in the countries of our geographical and cultural environment."

 (The grounds of the previous sentences are also included in the recent sentence of the TC No. 74/2022 of June 14 , which reproduces them and confirms their validity)

Judgment of the European Court of Human Rights of 12 January 2021 : The Court does not find it unreasonable for the State to consider it necessary to protect its general economic interest in raising public revenues by means of public scrutiny aimed at discouraging non-compliance with tax obligations. It considers that this purpose justifies interference in the rights to privacy and data protection of debtors, including the publication of their addresses. And it says expressly: " In the circumstances of the case, making the information in question public could not be considered a serious interference in the personal sphere of the applicant. It is not apparent that the publication of your personal data would impose a greater burden on your personal life than is necessary to satisfy the legitimate interest of the State ."

It should also be stressed that the sole legitimate purpose of the duty to contribute is to support public expenditure, that is, to provide sufficient means to cover the general needs that a social and democratic State such as ours must meet in its objective of achieving a more egalitarian and fair social order. This connection between the duty to contribute and its social purpose is what turns the individual duty to contribute of each citizen into a matter of general interest that can, in certain circumstances, make said duty prevail over the individual rights of taxpayers.

In other words, this general interest characteristic of the duty to contribute is the one that seeks to provide sufficient resources to make effective the statement of art. 9.2 of the Constitution: " It is the responsibility of the public authorities to promote the conditions so that the freedom and equality of the individual and the groups of which he or she is a part are real and effective; remove the obstacles that prevent or hinder its fulfillment and facilitate the participation of all citizens in political, economic, cultural and social life" .

And this real equality and social justice, which are essential to make the rights of citizens effective, become in turn, in a sort of perfect legal circle, the essential and guiding principles of both the distribution of the tax burden (fair tax system) and the destination of the income obtained through taxes (equitable allocation of public spending).