Utilitarianism is an ethical theory with a long and distinguished history. It is a species of consequentialism, the theory that tells us we should seek to act in ways that will maximize the goodness or value of the consequences of our actions. As G. E. Moore dramatically put it, “Our ‘duty’… can only be defined as that action, which will cause more good to exist in the Universe than any possible alternative.” What makes utilitarianism distinctive is its understanding of what goodness or value consists in, namely happiness or utility. 

Though many of the philosophers I most esteem and respect are utilitarians, I am not. There are several good reasons not to be a utilitarian. One, I think, fatal, problem is that a theory that tells us to perform at any given time “that action, which will cause more good to exist in the Universe than any possible alternative” is a theory that fails spectacularly to do what we want an ethical theory to do: offer some practical guidance in life. The Universe is just way, way too big, the future ramifications of at least many of your actions way too vast for us to have even the faintest idea what actions will cause more good to exist than any other, not just proximally but in the very very long term, from now to the heat death of the Universe.

Utilitarianism is often criticised for demanding too much of us, imperiously robbing us of any autonomy by seeking to control and direct every aspect of our lives. Really it has the opposite problem. It demands nothing of us. Entirely clueless as we are about the long-term consequences of our actions, any choice we make makes as much sense as any other. Utilitarianism is a fast track to nihilism. It's not that happiness is not important. But the Universe is above our pay grade.