Michel-Louis Guérard des LauriersDeceased

Michel-Louis Guérard des Lauriers (1898–1988) was a French Dominican theologian. He developed sedeprivationism, also known as the Cassiciacum thesis, which remains influential in part of the sedevacantist world (Wikipedia; Wikipedia: Sedeprivationism).

Écône and Cassiciacum. He taught at the Society of Saint Pius X seminary at Écône until 1977, when his appointment was ended over his position on the papacy. In May 1979 the first issue of the journal Cahiers de Cassiciacum set out his thesis. It holds that the post-conciliar popes were validly designated and so hold the papacy "materially" (materialiter), but lack its formal element, the authority to teach and govern, unless they withdraw the changes of Vatican II (Wikipedia: Sedeprivationism).

Episcopal consecration. Archbishop Ngô Đình Thục consecrated him a bishop in Toulon in May 1981, without a papal mandate. According to encyclopedic accounts, a 1983 Vatican finding declared that an automatic excommunication applied to him. He in turn consecrated Günther Storck and Robert McKenna. McKenna later consecrated Donald Sanborn, a present-day advocate of the thesis (Wikipedia; Wikipedia: Sanborn).

Debate. The thesis remains contested among traditionalists. A 2026 essay at Nuntiatoria, a site that describes itself as a traditional Catholic initiative, responds to a defence of the thesis by Fr. Federico Palma. It summarises how the thesis relies on an obex (an obstacle in the will) to explain why authority is not received, and argues, as its title indicates, that the thesis does not solve the problem it was designed to address.