Singer's work has attracted criticism from other philosophers. Bernard Williams, who was a critic of utilitarianism, said of Singer that he "is always so keen to mortify himself and tell everyone how to live". Williams criticised Singer's ethic by saying that he's "always so damn logical" and thus "leaves out an entire dimension of value". Williams claimed that Singer's utilitarianism is impractical as it's impossible to "make these calculations and comparisons in real life".


Williams develops an extended critique of Singer for suggesting that speciesism is a prejudice roughly equivalent to sexism or racism by suggesting that we have yet to face the sort of scenarios where species membership would become a morally significant property, but that some science fiction-style thought experiments may provide such examples. He imagines an invasion of aliens who are "very disgusting indeed: their faces, for instance, if those are faces, are seething with what seem to be worms, but if we wait long enough to find out what they are at, we may gather that they are quite benevolent". Said aliens "want to live with us—rather closely with us" even though their "disgustingness is really, truly, unforgettable". Williams also suggests that another sort of alien visitors might have "much more successful experience than we have in running peacable societies" but that they would need to exercise significant control and remove the cultural autonomy of human beings. In both scenarios, Williams argues, it would be perfectly reasonable for human beings to treat their species membership as a reasonable morally significant property. Singer responds to Williams by arguing that the right and courageous thing to do is to make the decision without regards to species.
ウィリアムズは、種差別主義は性差別主義や人種差別主義にほぼ匹敵する偏見である、と主張したシンガーに対し、さらなる批判を展開している。そこでウィリアムズは、種への帰属が倫理的に重要な属性となるようなシナリオに我々はまだ直面していないが、サイエンスフィクション的な思考実験がそうした事例を提供する、と述べている。そこで彼は異星人の侵略を想定しているが、その異星人は「本当に嫌悪感を催させる。例えば彼らの顔――もしそれが顔ならば――は虫のようなもので覆われている。しかし時間を掛けて彼らの目的を探ると、彼らは極めて善良であるという結論に我々は達する。」 その異星人は、「彼らの気持ちの悪さは本当に我々にとってまったく許容できない」にも関わらず、「我々と一緒に住みたい、我々と親密になりたいと考えている。」 ウィリアムズは別の異星人の来訪シナリオも提示しており、そのシナリオでは、異星人は「平和な社会を成功裡に運営する経験を我々より遥かに積んでいる」が、人類をかなりの程度管理して、人類の文化的自立性を除去することを求めている。いずれのシナリオにおいても、人類が自らの種への帰属を倫理的にそれなりに重要な属性として扱うのは完全に理に適っている、とウィリアムズは論じる。シンガーはウィリアムズに反論し、正しく勇気ある行動は種とは関係なく決断することだ、と論じている。


In a more recent critique, Jan Deckers uses a thought experiment that involves aliens once again to argue, like Williams, that speciesism makes good moral sense. Deckers argues that we should not only embrace speciesism, but also 'animalism' (an interest in attributing greater moral significance to either living or dead animals than to other biological organisms) and 'evolutionism' (an interest in ascribing greater moral significance to those animals who are biologically closer to us). Whereas Deckers shares with Singer a commitment to an interest-based ethic and to a theory that aims to maximise morally significant interests, his speciesist ethic also identifies a moral problem with the consumption of animals who die naturally or accidentally, a problem that is not recognised by Singer. In this way, this position provides a more coherent defense of qualified moral veganism that also question the consumption of in-vitro flesh.
Philosopher Roger Scruton wrote in 2000, "Singer's works, remarkably for a philosophy professor, contain little or no philosophical argument. They derive their radical moral conclusions from a vacuous utilitarianism that counts the pain and pleasure of all living things as equally significant and that ignores just about everything that has been said in our philosophical tradition about the real distinction between persons and animals".

ちなみに2日エントリではシンガーと日本の相模原殺傷事件との関わりを取り上げたが、シンガーのHPがリンクしている直近の著作では、シンガーが日本の別の問題に関心を寄せていることが示されている。そのLAタイムズの論説(Jordan Sosnowskiと共著)では太地町のイルカ追い込み漁を槍玉に挙げており、これをやめさせるために世界動物園水族館協会(WAZA)だけでなく国際海洋動物トレーナー協会(IMATA)も追い込み漁で得られたイルカの購入と手を切るべき、と主張している。