The principle 'ought implies can' holds that one cannot have a moral obligation to do something impossible. If you ought to do X, then it must be possible for you to do X. This principle constrains the scope of legitimate moral demands and connects obligation to human capacities. It rules out obligations to achieve the impossible but raises questions about the relevant sense of 'can' and whether all moral principles respect this constraint.
Test the principle: Can someone be obligated to run a four-minute mile? To achieve perfect virtue instantly? To love someone on demand? Where does 'can' fail to support the obligation?
Assuming 'can' means metaphysical possibility when it might mean practical or psychological possibility. Thinking the principle applies equally to all obligations. Overlooking that constraints on ability might themselves be subject to moral evaluation.
From your study of normative ethics, you know that moral claims come in different strengths: some are obligations, others are ideals or supererogatory acts. Ought implies can is a constraint on what can count as a genuine obligation in the first place. The principle states: if you morally ought to do X, then you must be capable of doing X. Obligations without capacity are incoherent — you cannot be blamed for failing to do the impossible.
The principle's force becomes clear through examples. No one has an obligation to stop an asteroid from striking Earth with their bare hands, even if the stakes are catastrophic. The impossibility of the action removes it from the domain of obligation entirely. But notice how quickly the interesting cases emerge: Can you have an obligation to feel grief? To believe something on command? To achieve perfect impartiality? These test whether the relevant "can" is physical, psychological, or normative.
The key philosophical challenge is specifying what kind of possibility the principle requires. Metaphysical possibility is the weakest threshold — something is possible if it could occur in some possible world. Practical possibility is tighter — the agent has the real-world means and opportunity. Psychological possibility is tighter still — the agent is capable of performing the action given their actual mental states. A moralist who says "you ought to love your enemy" may be invoking a psychological demand; a critic who invokes ought-implies-can is questioning whether love can be commanded at all.
A further complication: some constraints on ability are themselves morally evaluable. If someone becomes incapable of performing an obligation due to their own prior negligence — say, a driver who becomes intoxicated and thereby cannot safely drive someone to the hospital — their incapacity does not fully excuse them. The principle may apply to the moment of action while leaving intact a prior obligation not to become incapacitated. This "tracing" move is important for moral responsibility theory, which is exactly where this principle builds toward.
Finally, it is worth noting that not every ethical framework respects ought-implies-can equally. Kant invoked it to argue that the categorical imperative cannot demand the literally impossible. Consequentialists sometimes resist it, holding that the "ought" in "we ought to minimize suffering" can survive even when full compliance is unachievable — functioning as an ideal rather than a binding duty. The principle's scope, then, is itself a contested question in normative ethics.
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