The situation determines what one should do. J. A. T. Robinson says the old morality starts with general principles, supernaturally oriented, which sometimes take precedence over persons. The new morality begins with the particular situation and emphasises the priority of persons. Love, alone, because, as It were, it has a built-in moral compass, enabling it to 'home' intuitively upon the deepest need of the other, can allow itself to be directed completely by the situation. Joseph Fletcher says there are three approaches in making moral decisions, namely, the legal, the antinomian, and the situational approach.
Legalism enters every situation encumbered with prefabricated rules and regulations.
The letter of the law reigns. Principles, codified in rules are directives, not merely guidelines. Solutions are preset. You can look them up in a book. It may develop into a systematic drawing out of rules as to when rules may be broken.