The Stanford Encyclopedia of Philosophy
Revised June 30, 2014
Here is an excerpt:
What is common to the two well-known cases is conflict. In each case, an agent regards herself as having moral reasons to do each of two actions, but doing both actions is not possible. Ethicists have called situations like these moral dilemmas. The crucial features of a moral dilemma are these: the agent is required to do each of two (or more) actions; the agent can do each of the actions; but the agent cannot do both (or all) of the actions. The agent thus seems condemned to moral failure; no matter what she does, she will do something wrong (or fail to do something that she ought to do).
The Platonic case strikes many as too easy to be characterized as a genuine moral dilemma. For the agent's solution in that case is clear; it is more important to protect people from harm than to return a borrowed weapon. And in any case, the borrowed item can be returned later, when the owner no longer poses a threat to others. Thus in this case we can say that the requirement to protect others from serious harm overrides the requirement to repay one's debts by returning a borrowed item when its owner so demands. When one of the conflicting requirements overrides the other, we do not have a genuine moral dilemma. So in addition to the features mentioned above, in order to have a genuine moral dilemma it must also be true that neither of the conflicting requirements is overridden (Sinnott-Armstrong 1988, Chapter 1).
The entire page is here.
Editor's note: Anyone interested in ethics and morality needs to read this page. It is an excellent source to understand moral dilemmas as well as ethical dilemmas when in the role of a psychologist.