Much of how Catholic teaching appraises what is sin is based on the notion of specific acts being “intrinsically evil,” or being wrong by their very nature – despite whatever the circumstances or intentions may be. This is especially true with regards to Catholic teaching on sexual morality. For example, no matter what good intentions spouses may have, sexual acts that do not allow for the creation of new life are objectively evil. Similarly, no matter what goods may come from a homosexual relationship, every sexual act is inherently wrong. Some have criticized this outlook as physicalism, which judges actions mainly on the biological or physical aspects of the action.
Where does this moral outlook comes from? Is it primarily rooted in a certain philosophy the church has adopted? St. Thomas Aquinas? Etc.
Can the understanding of Catholic morality ever change, say, from a act-based to intention based? Or in some other way? At least in some cases? After all, not everything in Catholic morality is judged immoral by the act alone. Mutilation is not in itself wrong, but it can be. Not all killing is wrong – though that is also a physical act, too.