Today is the big day. This evening at 6 p.m., Carles Puigdemont will appear before the regional parliament to discuss "the general political situation" in Catalonia. Most of the country expects that to translate as some kind of declaration of independence from Spain.

The precise wording of that statement, and to what extent it will be interpreted as a genuine declaration of independence by courts, prosecutors, the central government and Catalan separatists themselves, is unknown this Tuesday morning.

The size and reach of the Spanish state's response to any declaration of independence...