You’re correct (I think) in seeing a tension - that uncertainty is one of the reasons I like Singer’s book so much. And I may not have explained my second-hand understanding of Singer well.
I meant to say that arbitrary=command: that “moral” strictures come from an outside source.
So the choice to be ethical, in the absence of outside authority: is it arbitrary? Not really, but kind of yes. One can base an ethics on basic beliefs like “I hate suffering” or “I love being white” or “Give what you own to the poor and follow me.” This choice is arbitrary, but we’ll all have reasons to follow, or not follow, each of these basic beliefs.
And once we make these choices, free of outside commands, we can develop ethics. Hopefully, they’ll be good ones.
The prohibition on recreational or performance enhancing drug use? Prohibitions on performance enhancing drugs doesn’t really make sense. Since so much of whether you can become an elite athlete is genetic, drugs help level the playing field and reach your biological potential. How exactly are they cheating? At the very least we should have sports that are drug free and sports where you can use any drug you want. What we have now is a situation where almost all the elite athletes use a wide range of drugs, but they cover it up.
Also the prohibition on recreational drugs makes little sense. Sure some drugs are addictive but many are not. And if they were legal, they would be dirt cheap ($1 per gram of highly pure drugs) which means people wouldn’t need to commit crimes to get them. People in general are not robbing homes and stealing purses to buy beer the way they do to buy meth or heroin. There are some valid moral prohibitions against them. They make you lose motivation and lose interest in friends, family and responsibility. A major reason I decided not to try recreational opiates is because opiates activate reward centers in the brain that are meant to be activated by socializing and friends/family. I fear this would make me lose interest in friends and family if I tried them.
But there are hundreds of recreational drugs, and many are not physically addictive. I guess the best moral guideline is ‘the cult of purity’. As humans we seem to have a need for a culture of purity. Since racial, religious or ethnic purity are no longer socially sanctioned, we condemn drugs instead. But the drug condemnation is in many ways just a proxy for racial and cultural purity. Blacks associated with cocaine, crack and marijuana. Mexicans with marijuana. Chinese and opiates. Hippies and hallucinogens. etc.
Circumcision for boys as morally acceptable. No real justification other than ‘everyone else does it’. However circumcision does have some health benefits, but those aren’t really the cause of why people circumcise. That is just an after the fact justification.
Condemning people for some poor lifestyle choices, but not others. Certain lifestyle factors it is ok to criticize people for. Sedentary lifestyle, poor diet, obesity, smoking. All these things increase morbidity and mortality.
But other lifestyle factors also increase morbidity and mortality. Poor sleep, lack of friends, being unmarried, working long hours. As a society we really do not feel it is acceptable to criticize others for not having friends the way we criticize them for smoking and being sedentary.
Also moderate alcohol consumption increases health. Why do we not nag people to do that the way we nag them to exercise? Both situations involve asking someone to engage in behavior they normally wouldn’t to extend their lifespan and healthspan.