The Expanding Circle (1981) is one of Peter Singer’s best-known books. In it he argues that moral progress is marked by a gradual widening of the circle of beings whose interests we are willing to value similarly to our own. The circle of altruism keeps expanding to encompass dogs and pigs and great apes and dolphins and other animals. His carefully reasoned arguments made him the philosopher of the animal liberation movement.

However, in a challenge to key elements of Singer’s philosophy in the Practical Ethics blog, Joseph Moore contends that recent developments in philosophical psychology have left Singer’s theories in the dust. Singer is fundamentally a “hedonist”, meaning that the principal substantial interests that morally relevant beings have are pleasure and pain. He claims that “the capacity for suffering and enjoyment is a prerequisite for having interests, a condition that must be satisfied before we can properly speak of interests at all”.

This, writes Moore, is utterly passé. Restricting “interests” to sentience ignores biology. “…the philosophy and ‘positive’ psychology of well-being has exploded in the intervening decades. There are now many more views under discussion, and relatively few expert participants in these discussions nowadays are hedonists. There are plenty of other purely psychological views, as well as more ‘objective’ (not-purely-psychological) views. If we accepted any of these alternatives, we would thereby draw the line of interests and moral concern in different places from Singer’s line of sentience (i.e., capacity for pleasure and pain).”

The upshot of this is that Singer stands accused of the speciesism that he critiques in society.

The supposed necessity of sentience for having interests is why Singer limits his ‘principle of equal consideration of interests’ to (some) animals and does not extend it to living things in other kingdoms—plants, fungi, bacteria, etc.—or other kinds of subjects. But this relatively undefended assertion was dubious in 1975 and is even more dubious now. Singer’s restriction of interests to sentient beings is just as arbitrary as the speciesism he decries.

In Moore’s estimation, Singer is right to be concerned about animal welfare. But he should be open to considering the interests, rights and welfare of the plant world as well.  Down with “zoocentrism” is the theme of Moore’s critique.

(To be honest, I’m not sure if Moore is completely serious in attacking Singer. He might be a mischievous Aristotelian trying to discredit the world’s leading animal rights philosopher by reductio ad absurdum. But it is a thought-provoking comment.)