"Ought" Implies "Can"

Many philosophers use the principle "Ought implies can" as a basic test of moral obligation.

If something is a moral obligation (a duty), then we ought (should) do it.

The importance of this idea might not be obvious. However, by a basic rule of logic called contraposition, we can turn the principle around, into this second principle: "Can't implies ought not." In other words, if you can't do x, then you have no duty to do x. (And "can't" means you really can't do x because x is impossible, not just that you don't like the results of x.)  It also means that we should not blame people for failing to do what they cannot possibly do. 

Seen in this way, the principle tells us that our obligations are restricted to what is humanly possible. The real world puts a limit on ethical responsibility.

For example, if a kitten is stuck in a tree, then you would be wrong to insist that Pat has a moral obligation to levitate into the air and rescue the kitten. Being human, Pat cannot levitate, so Pat has no such obligation. To take a more realistic example, if there are so many patients suffering from a fatal disease that doctors cannot treat them all, then doctors have no moral obligation to treat them all.

Another implication of the principle is that changes in real-world circumstances can cancel some obligations. 

For example, if I promised to meet you for lunch, then I have an obligation to do so. But if I'm driving to meet you for lunch and I get into an accident, I get knocked unconscious, and the paramedics throw me in an ambulance and drive me away, so that I cannot meet you for lunch when I promised, then I did nothing wrong. This implication leads some to object that we can remove obligations by purposely getting ourselves into a situation where we cannot do what we are obligated to do (e.g., the court orders Frank to pay child support, and Frank goes to Vegas and loses every cent he has by continuously betting on red at the roulette wheel -- since Frank now cannot pay the child support, Fred has no obligation to do so). But this objection gets it wrong. If x is an obligation for person a, but x depends on doing y, then if y is under a's control, then a has an obligation to do y. (BECAUSE Frank was obligated to pay child support, he was therefore obligated NOT to go to Vegas and gamble away the money!)

Do not confuse "x is impossible" with the closely related question of x's having morally objectionable consequences. That issue is covered by the Doctrine of Double Effect. 

Do not confuse "ought implies can" with "can implies ought." The fact that one can do something is no evidence that one should do it. (I can cause disgust by teaching class while eating fried chicken feet. It does not follow that I should eat fried chicken feet while teaching class.) One of the reasons that we engage in ethical thinking is to choose among our options.

BACKGROUND: Immanuel Kant seems to have been the first philosopher to explicitly formulate the principle: "since reason commands that such actions should take place, it must be possible for them to take place" (Kant, Critique of Pure Reason, A807/B835, Kemp Smith translation)