Summary: If you object to consequentialist ethical theories because you think they endorse horrible or catastrophic decisions, then you may instead be objecting to short-sighted utility functions or poor decision theories.

Recommended: Decision Theory Paradox: PD with Three Implies Chaos?

Related: The "Intuitions" Behind "Utilitarianism", Yvain's Consequentialism FAQ

The simple idea that we ought to choose actions according to their probable consequences, ever since it was formulated, has garnered a rather shocking amount of dissent. Part of this may be due to causes other than philosophical objections, and some of the objections get into the metaphysics of metaethics. But there's a fair amount of opposition on rather simple grounds: that consequentialist reasoning appears to endorse bad decisions, either in the long run or as an effect of collective action.

Every so often, you'll hear someone offer a reductio ad absurdum of the following form: "Consider dilemma X. If we were consequentialists, then we would be forced to choose Y. But in the long run (or if widely adopted) the strategy of choosing Y leads to horrible consequence Z, and so consequentialism fails on its own terms."

There's something fishy about the argument when you lay it out like that: if it can be known that the strategy of choosing Y has horrible consequence Z, then why do we agree that consequentialists choose Y? In fact, there are two further unstated assumptions in every such argument I've heard, and it is those assumptions rather than consequentialism on which the absurdity really falls. But to discuss the assumptions, we need to delve into a bit of decision theory.

Consequentialists see morality through consequence-colored lenses.  I attempt to prise apart the two concepts to help consequentialists understand what deontologists are talking about.

Consequentialism¹ is built around a group of variations on the following basic assumption:

• The rightness of something depends on what happens subsequently.

It's a very diverse family of theories; see the Stanford Encyclopedia of Philosophy article.  "Classic utilitarianism" could go by the longer, more descriptive name "actual direct maximizing aggregative total universal equal-consideration agent-neutral hedonic act² consequentialism".  I could even mention less frequently contested features, like the fact that this type of consequentialism doesn't have a temporal priority feature or side constraints.  All of this is is a very complicated bag of tricks for a theory whose proponents sometimes claim to like it because it's sleek and pretty and "simple".  But the bottom line is, to get a consequentialist theory, something that happens after the act you judge is the basis of your judgment.

To understand deontology as anything but a twisted, inexplicable mockery of consequentialism, you must discard this assumption.

Deontology relies on things that do not happen after the act judged to judge the act.  This leaves facts about times prior to and the time during the act to determine whether the act is right or wrong.  This may include, but is not limited to:

• The agent's epistemic state, either actual or ideal (e.g. thinking that some act would have a certain result, or being in a position such that it would be reasonable to think that the act would have that result)
• The reference class of the act (e.g. it being an act of murder, theft, lying, etc.)
• Historical facts (e.g. having made a promise, sworn a vow)
• Counterfactuals (e.g. what would happen if others performed similar acts more frequently than they actually do)
• Features of the people affected by the act (e.g. moral rights, preferences, relationship to the agent)
• The agent's intentions (e.g. meaning well or maliciously, or acting deliberately or accidentally)