Assuming that moral rules can and do have exceptions the problem becomes one of understanding the nature of the logical relations between exceptions and rules. One way to conceive this relation is to think of exceptions as cases which fall outside and restrict the scope of a rule. We may speak of exceptions conceived in this way as exceptions 'toll a rule. Another way of conceiving the relation is to think of exceptions as built in negative specifications of the rule. In this sense an exception is part 'of' the rule not a case that limits it so to speak.
Singer appears to adopt the first way of construing exceptions.
Baier appears to adopt the second. According to Singer there is a considerable difference between moral principles and moral rules. Moral principles he says hold in all circumstances, allow no exceptions and are always relevant, whereas rules hold only generally, do allow exceptions and are not always relevant. Furthermore principles are apt to be more abstract than moral rules, though he says, they are not necessarily less definite. It is by appealing to principles that moral rules as well as exceptions to them are justified and established.