[Code of Canon Law]
"No doctrine is understood as defined infallibly unless this is manifestly evident."
749 Section 3
[Fr. Francis Sullivan]
"The 'primary object' of the magisterium consists of truths which are part of the 'saving truth and rule of conduct' of which the Gospel is the source; it is only such truths that can be defined as dogmas."
Creative Fidelity, pg. 42.
[Fr. Francis Sullivan]
"Now the gospel here does not refer to a written book, but to the whole message of Jesus Christ, the Word of God, and to the entire saving and revealing event of his incarnation, ministry, death, and resurrection...this 'saving truth' is contained in written books (the scriptures) and also in 'unwritten traditions."
Creative Fidelity, pg. 41.
[Vatican I, Pastor Aeternus]
we teach and define as a divinely revealed dogma that when the Roman Pontiff speaks EX CATHEDRA, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals.
Chapter 4, Section 9
[Vatican I, Pastor Aeternus]
...the Holy Spirit was promised to the successors of Peter not so that they might, by his revelation, make known some new doctrine, but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
Chapter 4, Section 6
[Fr. Francis Sullivan]
Vatican I defined that the pope speaks infallibly under the following conditions: "when he speaks ex cathedra, that is, when, acting in the office as the shepherd and teacher of all Christians, he defines, by virtue of his supreme apostolic authority, a doctrine concerning faith or morals to be held by the universal church...There is, in fact, general agreement that the definitions of the immaculate conception by Pius IX and that of the assumption by Pius XII meet all the conditions for defining dogmas of the faith...However, when we look to the past and attempt to determine which papal statements in the course of history have met the conditions for dogmatic definitions, matters are much more complicated. First of all, while it is hardly likely that a modern pope would use the term 'we define' other than in a solemn definition, or fail to use it in such a definition, one cannot presume that this term always had such a specific meaning in documents of the past. In older documents, neither the presence of the term 'we define,' nor it's absence, would be decisive for determining the intention of a pope to speak ex cathedra...the fourteenth century Carmelite bishop, Guido Terreni, who was the first to attribute infallibility to the papal exercise of teaching authority, insisted that the pope would teach infallibly only when he determined a matter of faith for the whole church after having consulted the college of cardinals...The idea that a papal statement should be recognized as a dogmatic definition by the way it has been received, and by the fact that it has been decisive for the faith of the church, might seem to be in conflict with the statement of Vatican I that a papal definition is "irreformable of itself, and not from the consensus of the church." To this objection one can reply that the pope can define as dogma only a truth that has been revealed, and that must therefore be contained at least implicitly in the faith of the church. The eventual failure of any papal doctrine to be received by the church as an article of it's faith would show that that doctrine was not contained in the deposit of faith, and hence was not capable of being defined as dogma.
Source: Fr. Francis Sullivan "Creative Fidelity" Weighing and Interpreting Documents of the Magisterium." Pgs. 80-82, 88.