Agents have aims. Any aim can be either simple or complex. If an aim is complex, then its different components make irreducibly different demands on the agent. The agent cannot rationally respond to all these demands by promoting all her different component aims at once. She must recognise a distinction between the rational response to any component aim of promoting it, and the rational response of respecting it.

If the goods are incommensurable, then rational agents have complex aims.

So if the goods are incommensurable, rational agents do not only promote whatever aims they recognise.

But consequentialism tells agents only to promote whatever aims they recognise.

So if the goods are incommensurable, consequentialism is wrong.

I note applications of this argument to the writings of Robert Nozick, Philip Pettit, and John Harris.