Synopsis This dissertation examines one of the most basic principles of taxation of our tax system: the principle of taxation of the real income. Enshrined in the Constitution of the Portuguese Republic in 1976, represents the essential core of ’70s tax reform. Nearly 40 years after its constitutional consecration, it becomes necessary to study this principle, exempt from the markedly revolutionary character on its inception, and, accordingly to the changing times, its respective goals and its limits. In this context, it especially matters to verify if the income obtained by the taxpayer is effectively determinable, or, failing that, if the income on which the tax has focused still respects the limits and purposes intended by the principle of taxing the real income. Finally, in reflex of the carried out analysis, some considerations cannot fail to be made about the autonomy of that precept against the principle of the ability to pay and relevance of its constitutional enshrinement.
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