on their interpretation of modernism as being a heresy, this is also debated. Positions within sedevacantism On clergy, Mass, and sacraments Some sedevacantists accept the consecrations and ordinations of sedevacantist bishops and priests, and the offering of Masses and the administration of sacraments by the said bishops and priests, to be licit because of epikea, i.e. "the interpretation of the mind and will of him who made the law". In this case, the ecclesiastical laws (e.g. prohibition of consecrations of bishops without papal mandate; prohibition of administration of sacraments without ecclesiastical authorization) are interpreted to cease when to follow them would be impossible, harmful, or unreasonable, or would mean transgressing divine laws (e.g. the church must have bishops and priests; Catholics must attend Mass and receive the sacraments), and because of a historical precedent for consecrating Catholic bishops during a long vacancy of the Holy See. On liturgy Another divisive question among sedevacantists is whether it is permissible to go to "una cum" masses (Traditional Latin Masses where the name of the person considered by mainstream Catholics as Pope is spoken in the Roman Canon, specifically in the "Te igitur" prayer, where the priest says "una cum famulo tuo Papa nostro N." ["together with Thy Servant N., our Pope"]). Such "una cum" masses are offered by the priests of the Priestly Fraternity of Saint Peter. Some argue that it is, or may be, permissible, while others argue that it is not permissible, and that such masses are illicit and forbidden to Catholics. Relationship to sedeprivationism A sizeable portion of sedevacantists affirm the Thesis of Cassiciacum of the Dominican theologian Bishop Michel-Louis Guérard des Lauriers as being a valid position, which states that John XXIII and his successors are popes materialiter sed non formaliter, that is, "materially but not formally," and that the post-Vatican II popes will become pope if they recant their heresies. Such position is endorsed by the Istituto Mater Boni Consilii and the Orthodox Roman Catholic Movement. Demography There are estimated to be between several tens of thousands and more than two hundred thousand sedevacantists worldwide, mostly concentrated in the United States, Mexico, Canada, France, the United Kingdom, Italy and Australia, but the actual size of the sedevacantist movement has never been accurately assessed. It remains extremely difficult to establish the size of the movement for a wide range of reasons, such as the fact that not all