Is the Catholic Church flexible enough to change? If so, what can change and how? The Catholic Church teaches with authority, but how far does that authority go? What can the fathers of the Church change, and how? Certainly matters of discipline can be changed. Bishops can mandate a Friday fast from meat or  not. The Church can dispense a married man from the vow of celibacy in order to be ordained. These, and many other matters of discipline may be altered.

The doctrinal and moral teaching of the church, however, cannot be altered. Nevertheless, our understanding of the unchanging doctrines and moral teaching of the church may develop. Because of this, in today’s church some Catholics and Anglicans argue for radical change in the church’s ministry, the ecclesial structures, or the church’s understanding of gender roles, the definition of marriage and sexual morality because they sincerely believe in the ‘development of doctrine’. So they argue: “The church used to teach that usury was a terrible sin. Now it is accepted. The church used to be in favor of slavery. Now it isn’t.” These are sloppy assertions–vague generalizations. In fact, the church teaching on usury past and present is far more subtle and nuanced than that. The church was against greedy and avaricious usurers–and that is still her teaching today. Here’s a page that explains it. Was the church ever ‘in favor of slavery’? Some popes may have owned slaves, and the actual behavior of Catholics was inconsistent, but there is no official church teaching which formally condones or commends slavery, instead, the Catholic Church, from the earliest times has been one of the few voices to condemn slavery. Go here for an article from an Evangelical journal showing how the Catholic Church has been in the forefront of opposition to slavery, and go here for a more detailed history of the issue.
But I am getting off track. While these two examples are specious, they are also an attempt to show not development of doctrine, but an ‘about face’ by the church. It is still true that doctrine does develop and we have Blessed John Henry Newman to thank for thinking this through and setting down seven ‘notes’ on the development of doctrine.  Newman’s seven notes of a valid doctrinal development are (1) preservation of type, (2) continuity of principles, (3) assimilative power, (4) logical sequence, (5) anticipation of its future, (6) conservative action on its past, and (7) its chronic vigor.  All seven of these characteristics need to be present for a doctrinal development to be valid.
The notes can be best understood with the analogy of an acorn growing into an oak tree.  Newman’s first note is ‘preservation of type.’ This just means that a validly developing doctrine does not become something essentially different. The oak tree was in the acorn all along, even though the oak tree and the acorn look completely different. On these terms one might argue that monogamy was a natural development from polygamy because it was a refinement and a focussing of what was already present in polygamy–the idea of a man and woman being faithful to one another in a marriage contract.  However, Newman’s first note would exclude homosexual marriage from being a valid development because two men marrying one another is not a development from the essential idea of marriage, but a radical deviation. 
The second note is ‘continuity of principles’. That is to say, the underlying principles of the doctrine do not change although they may flower and grow and develop. So, for example, the underlying principle of Marian beliefs is that Mary is a pure Virgin full of grace. This principle or abstract underlying belief is expounded and clarified with a developed doctrine like the Immaculate Conception. The underlying principle of marriage is the natural complementarity of the sexes within the created order for the purpose of procreation. This understanding may flower and grow (as it does in John Paul II’s Theology of the Body) but the underlying principle does not change–as it would, for example in homosexual marriage.
Newman’s third note is ‘assimilative power’. This simply means that Catholic doctrine and teaching is alive and growing. It does not reject all innovation and fresh ideas, but studies them carefully to take on and assimilate that which is good and true as it is discovered. So, for example, with the debate on women’s ordination the Catholic Church may develop new understandings and acceptance of true femininity, women’s roles, thus gleaning what is good and positive from feminism. Likewise, the Catholic Church may study the rise of homosexualism with an objective eye and assimilate new understandings of same sex attraction and the value of  celibate friendship. 
The fourth note is ‘logical sequence’. The development must develop logically from it’s antecedents, and not be a logical contradiction of them. This is similar to the second note on continuity of principles, except that it looks at the actual development itself and tests it to see if the development is logical or illogical. Whether it is logical or not, of course, depends on your understanding of the core doctrine or moral teaching to start with. If, for example, you believe that marriage is only about the personal experience of  ‘love’ and ‘commitment’ and nothing else, then to extend marriage to homosexual persons might be a logical development. If, however, you understand marriage as a contract between a man and a woman for the procreative and unitive functions, then the development is illogical.
The fifth is ‘anticipation of it’s future’. This means that the development could be seen naturally within the earlier doctrine or practice. For example, the dogma of the Immaculate Conception is naturally anticipated in the much earlier dogma of the perpetual virginity of the Blessed Virgin Mary. The Immaculate Conception is simply a deeper understanding of the already existing idea that Mary was kept pure from the stain of sin. In this sense the development can be seen to be anticipated.
The sixth: ‘conservative action on it’s past’. The development must conserve the past, not destroy it. That is to say, that while the development enhances and clarifies and expands the past, at the same time it conserves the past. So when considering whether a doctrinal development is valid we ask whether it destroys what went before.  If it does, it is a corruption not a development.
The seventh note is ‘chronic vigor’. This is simply the idea that true ideas stand the test of time. They prevail. Corruptions, heresies and half truths die out. 
I’m well aware that in the debate over innovations in the church Newman’s notes, rather than giving all the right answers, often only ask the right questions, but right answers cannot be arrived out without the right questions first, and this brief outline might help all of us who engage in such debates.