Sedeprivationism is a term coined by William J. Morgan, an English layman and a sedevacantist apologist. The word sedeprivationism is an amalgamation of two words, sede and deprivation, into a single descriptive term. Individuals or groups who follow the sedeprivationist school of thought, such as the Istituto Mater Boni Consilii and others, hold that while the current occupant of the papacy is a duly elected pope, unless he recants the changes brought by Vatican II, he lacks authority to either teach or govern.
Morgan developed the term in order distinguish it from sedevacantism, a position which he held himself. He applied the term to one of the Catholic theological explanations for the traditionalist opposition to the conclusions of the Vatican II council. It holds that the Novus Ordo popes have been defective popes.
According to Matthew, sedeprivationists hold the view of the late Dominican theologian, Mgr Guérard des Lauriers, that John Paul II (like his Conciliar predecessors) was "materially 'Pope' only". That is, that Karol Wojtyla was the valid electee of the October 1978 conclave, though he has never actually become Pope, but, equally, has never forfeited his canonical claim on the Papacy.
This analysis follows the principles of the late French theologian, Bishop Michel Louis Guerard des Lauriers, O.P.. Bishop Guerard des Lauriers presented his thesis in a publication with the title Cahiers de Cassiciacum and therefore called commonly the Cassiciacum Thesis which states that the See of Peter, is not obtained and held in conformity with one of two prescribed requirements of a legitimate papal election by the Novus Ordo popes. The two mandatory requirements are:
- That the pope is elected legitimately by valid designated electors. This aspect designates the papal candidate as materially elected and designated candidate to the office of pope.
- That the newly chosen pope-elect expresses his acceptance and that on giving his assent he receive from Christ the form of the papacy i.e. the indefectible power or authority promised to St. Peter and his successors by which the elected candidate formally becomes pope and actually takes hold of the office of the papacy.
According to Matthew, they hold that both of these aspects are required and should any candidate fail in either one, then he does not hold the office of pope. The Catholics of the sedeprivationist school hold that all claimants of the papal office from at least Paul VI through Francis (the Novus Ordo popes) are invalid and that they do not hold the papal office except by right of designation due to a failure to receive the form of the papacy (i.e. the Authority) because his acceptance is impeded by a defective intention arising from their manifest disposition of apostasy; further, that a valid papal election has indeed transpired.
Sede vacante (i.e. the seat being vacant) technically refers to the period between a papal death and a newly elected papal candidate. During the interim period, the period of time when the papal seat was vacant, the chamberlain, the Cardinal Camerlengo, administered the Church in lieu of the pope. Sedevacantism is the notion of some Catholics that in this time of the Novus Ordo popes the state of the papacy is that of sede vacante. Sedeprivationism seeks to clarify and distinguish between sedevacantism, a simple and complete vacancy, and the more complex current state of affairs in Church in which there is an elected successor to Peter though deprived of the reception of the Authority of the Office. For illustrative purposes one might add another term: sedeplenist, to signify the mind of those Catholics like the SSPX who believe the Novus Ordo popes are full fledged popes truly in possession of the full Authority promised by Christ to St. Peter and his successors yet have substantially erred in the teaching of the universal Ordinary Magisterium on matters of faith and morals. In their perspective they believe this kind of error is possible to happen and that nowadays it has and so they may lawfully recognize and resist the fully seated Papal Authority on the most essential matters which constitute a religion : doctrine, morals, liturgy, sacramental system, code of law, and nature of government.
According to Lauriers’ thesis, Paul VI, John Paul I, John Paul II and (implicitly) Benedict XVI and Francis were or are defective popes in that, due to their espousal of the "modernist heresy", their consent to become pope was faulty or defective, so that they became legally designated to be pope, yet deprived of attaining full succession to the Authority of Peter.
Bishop Guerard des Lauriers’ thesis holds that the Vatican II popes succeed as legal designees to the papacy and continue the line of St. Peter materially. This means that the Vatican II Novus Ordo popes are legitimate designees to be true popes but not formally and thereby lack jurisdictional authority because of the obstacle that they posit to the reception of the authority. They are not the authority, they are not true popes or true bishops, but are legally in the position to become true popes and bishops, if they should remove the obstacle to the reception of the authority of the office.
Because the power of designation to office pertains to the purely legal and material side of authority, the Novus Ordo popes possess the power to legitimately designate to positions of power, until such time as this power is legally removed from them. As a result, there is a material hierarchy in place, i.e., someone legally nominated to be a pope, and others legally nominated to be bishops, and others legally nominated to be electors of popes, but none of these has any jurisdiction, and obedience is owed to none of them.
Because they lack the authority, which is the form which makes them to be what they are, Bergoglio is a false pope and the bishops are false bishops. The cardinals are true electors, to the extent that they are legally nominated to be designators of the pope. But their role pertains to the material order of authority, the order of designation only." 
- There is no real sede vacante state in the Novus Ordo popes since a man fills the role of potential pope;
- If the current potential pope recants from Modernism and returns to Catholicism, he will complete the process and attain to the fullness of the papacy — all else being equal.
- Arguably, the feud between those who say the Chair of St. Peter is totally empty (sedevacantists) and those who say it is totally full (sedeplenist, e.g. SSPX) can come to an end as the answer is neither of these two explanations as they compromise either the perpetuity of succession or the doctrine of idefectibility and infallibility.
The terms sedeprivationism and sedeprivationist were coined by the late English Catholic William J. Morgan.
- "Who We Are", Istituto Mater Boni Consilii
- "A few ... hold the view of the late Dominican theologian, Mgr Guérard des Lauriers, that John Paul II (like his Conciliar predecessors) is 'materially "Pope" only'. That is, that Karol Wojtyla is the valid electee of the October 1978 conclave, though he has never actually become Pope, but, equally, has never forfeited his canonical claim on the Papacy. — Such sedeprivationists ..." “Morgan’s document”
- “Through an impediment of the moral order, however, they cannot receive papal power who posit a certain moral obstacle, both voluntary and removable, for example, the refusal of episcopal consecration, or the intention of teaching errors or of promulgating harmful general disciplines, or the refusal of baptism in the case of the election of a catechumen (for example, St. Ambrose, elected to the episcopal see of Milan) These are capable of a valid designation because the impediment is removable, but the authority cannot be infused by God until the impediment is removed. The reason is that they are not capable of promoting the common good to the extent that they do not remove the obstacle. And because the impediment is moral and voluntary, the obstacle is reduced to an absence of the intention of promoting the common good. God, therefore, who is Subsistent Good, is not able to infuse power in him who posits a voluntary impediment toward the promotion of the common good.” "The material Papacy – Sodalitium". www.sodalitiumpianum.com (in Italian). Retrieved 2018-03-07.
- ”...Through an impediment of the moral order, however, they cannot receive papal power who posit a certain moral obstacle, both voluntary and removable, for example, the refusal of episcopal consecration, or the intention of teaching errors or of promulgating harmful general disciplines,...because the impediment is moral and voluntary, the obstacle is reduced to an absence of the intention of promoting the common good. God, therefore, who is Subsistent Good, is not able to infuse power in him who posits a voluntary impediment toward the promotion of the common good.” "The material Papacy – Sodalitium". www.sodalitiumpianum.com (in Italian). Retrieved 2018-03-07.
- Cf. Ab. Lefebvre : "Letter to future Bishops," wherein he either wittingly or unwittingly implies a similar distinction, in that the Novus Ordo occupants of the posts of authority in Rome by their elected presence qualifies them to some claim of succession yet are functionally defective to such an extent that one is obliged in conscience to refuse subjection to them on most essential matters. He just never identified precisely in what that substantial defect consists but clearly here believes they are absent of the Indefectible power inherent with the Office. He knew he had to protect Perpetual Succession and at the same time condemned them to be so defective as to make them antichrists.”
- Explanation of the Thesis
- Istituto Mater Boni Consilii (Institute of the Mother of Good Counsel)
- The Material Papacy, by Father Bernard Lucien, formerly of the Istituto Mater Boni Consilii
- Explanation of the Thesis of Bishop Guerard des Lauriers, by Bishop Donald Sanborn, rector of Most Holy Trinity Seminary, Brooksville, FL
- On Being Pope Materially, by Bishop Donald Sanborn, rector of Most Holy Trinity Seminary, Brooksville, FL
- De Papatu Materiali Pt. 1, by Bishop Donald Sanborn, rector of Most Holy Trinity Seminary, Brooksville, FL
- De Papatu Materiali Pt. 2, by Bishop Donald Sanborn, rector of Most Holy Trinity Seminary, Brooksville, FL
- The Problem of Authority in the Post-Conciliar Church: The Cassiciacum Thesis, by Fr. Bernard Lucian, translated by Rama P. Coomaraswamy, M.D.
- The Material Papacy, by Bishop Donald Sanborn, rector of Holy Trinity Seminary, Brooksville, FL
- Pope, Papacy and the Vacant See, by Fr. Francesco Ricossa of the Istituto Mater Boni Consilii