It is worth articulating the different ways in which philosophers disagree as this will help us better analyze and assess competing theories. Sometimes philosophers disagree about the facts. For instance, two philosophers might share the same basic normative theory but disagree about relevant features of the world. Suppose two philosophers agree that what matters morally is to make people as happy as possible. However, one believes that, psychologically speaking, what actually makes people happy is ensuring their safety, while the other believes that happiness depends on maximizing people’s freedom. Both agree that happiness is a particular emotional state, but they disagree about the facts about what causes it. Notice that if they both really care about doing the right thing, they are probably going to want to look at some empirical work here. For example, one might consider research in social psychology to see what really does make people happy.
Another possibility is that they disagree about what happiness means or, alternatively, what type of happiness is morally relevant. One might think that true happiness is an emotional state that is experienced moment to moment while the other might think that true happiness depends on achievement and overcoming various struggles and obstacles over a lifetime. This brings us back to philosophy. These philosophers are effectively disagreeing about what a certain concept means. Scientific investigations are unlikely to be helpful. Even if one thinks that the concept in question is empirically tractable, suggesting we can discover what happiness is with a properly designed experiment, they are still going to have to convince the other person that science can tell us this, which bring us back into the realm of philosophy. Basically, if you are trying to rationally convince someone of something that goes beyond the agreed-upon facts, you are likely doing philosophy.
Finally, we might simply accept different moral theories or rank them differently in importance. One might think that maximizing happiness is the single most important moral goal while the other thinks it is irrelevant because freedom is the only thing that matters morally, whether it makes you happy or not. Here again, there is philosophical work to be done and they will try to convince each other.
Notice that if we agree about the facts, moral concepts, and the applicable moral theory we should agree about the right course of action. If we are reasoning carefully and disagree about the right course of action it is almost certainly because we disagree about the relevant facts, the meaning of moral concepts, or the relevant moral theories.
Importantly, whatever we decide to do, we are morally responsible for the outcome of that decision—good or bad. We should expect to be held accountable for our actions. Happily, if we have carefully considered our options, listened to and learned from those who disagree, and looked at the situation through each ethical lens and from all relevant perspectives, we can expect to have a robust and convincing justification for our actions.
In applied contexts, there is the possibility that even if we disagree about the facts, the interpretation of moral concepts, and the correct normative theories we may nonetheless agree about what the right action is in a given situation. This gives us another reason for not just choosing one moral theory over the others but instead taking a more pluralistic approach. If we can show that the same action is required by a broad set of very different moral theories, then this becomes very powerful evidence that the action is morally required. So, even if you are inclined to think that one of the approaches discussed below is right to the exclusion of the others, you may be able to provide far more compelling arguments if you notice when theories agree.