The Catalan referendum raises two issues.
First of all, there is an issue concerning legality and democracy – the Spanish government alleged that the referendum went against the Spanish Constitution. Secondly, there is an issue concerning independence and self-determination – many think that Catalans are entitled to independence, or to a greater autonomy, and the referendum was apparently aimed at eliciting a plebiscite on that.
These two issues generate a further puzzle. If the referendum is not legal, then Catalan independence, if and when got, is not a new constitutionally enforced legal order – a state entity dividing up or implementing autonomous spheres within it. Rather, it is a genuinely unilateral and non-consensual secession. (Consensual secessions are possible – think about Norway seceding from Sweden in 1905, or the Supreme Court of Canada’s verdict on the possibility of a legitimate secession of Quebec.)
Some authors, Cass Sunstein among them, claim that a right to secession contradicts the very spirit of constitutionalism, as it encourages dissenting minorities to secede or to use the prospect of secession as a threat against majorities. Allen Buchanan suggested allowing a constitutional right to secession, provided that its implementation is constrained by supermajoritarian restraints. In recent political and moral philosophy, there is a discussion on the ethics of secession, namely on the moral reasons favoring secession. The best theories of the right of secession, as we will see, do not justify Catalonia's aspirations.
Some authors assimilate the right to secession to the right to revolution – or to the right to rebel in front of extreme and systematic violations of basic human rights Allen Buchanan includes among basic human rights the right of a minority to legal procedures of self-determination, compatible with the national sovereignty. In such an approach, secession is a sort of extreme measure against gross and persistent injustices. Secession is a remedy for previous injustice.