Have a look at the Strengths and Weaknesses of Kant's Ethical Theory.
All deontological (duty or rule-based) systems will have problems when two rules come into conflict. It is possible to have a third rule (Always tell the truth unless doing so endangers someone’s life), but this complicates the theory, resulting in rules with lots of clauses and sub-clauses (a little like our legal system).
There could also be literally millions of rules that are not self-contradictory but, if universalised, would seem absurd. Here Kant says that we should reject those rules which, if universalised, would produce a state of affairs utterly objectionable to all rational people. This suddenly looks less convincing than before – how can we tell what rational people would find objectionable? As with Utilitarianism, many philosophers prefer to adapt Kant’s theory rather than discard it completely.
Ross adapted the Kantian approach. He described our obligations as 'Prima Facie' duties. This means that they are, 'at first appearance', things that we must do. Just like Kant, he might say that we have an obligation not to kill, steal etc. In fact, he lists our obligations as follows, although this is not an exhaustive list:
- Duties of fidelity.
- Duties of gratitude.
- Duties of justice.
- Duties of beneficence to others
- Duties of self-development.
- Duties not to injure others
So, Ross may actually agree with a utilitarian that we have a duty or obligation to bring about the greatest good! This doesn't sound very Kantian at all, but wouldn't we will universal laws that brought about the greatest good?
So, we have obligations, but Kant said these were absolutes, Ross disagrees. They appear to be absolute (prima facie), but if two of them contradict, we clearly cannot honour both obligations. We need to determine which is the greater obligation, and then we have an absolute duty to follow that.
For example, I have promised to stay with a colleague's class while he makes an urgent phone call. I am clearly obliged to honour my promise. Someone rushes in and announces that a student is dying next door, and I am the only one who can save her (I have a First Aid Certificate). I clearly have a duty to save the student's life (save the cheerleader, save the world...). So, which duty am I obliged to honour? Ross would say I have a prima facie obligation to keep my promise, but an absolute obligation to save the student.