The greatest problem with a utilitarian view of animal rights is that it is based to a great extent on guesswork and personal opinion. Peter Singer himself acknowledges this when he discusses how recently, organizations such as the American Veterinary Medical Association began to take notice of the suffering of animals as an important factor in their treatment. How much does a pig suffer from being slaughtered? How much does a dairy cow suffer upon being separated from her calf? How much suffering is involved in the production of veal, or chicken, or eggs? All these appear to be questions which are impossible to answer with any degree of precision or objectivity, and Singer's view has often been attacked on this basis.

A rights-based approach seems even more fraught with difficulty. How would one ascertain what rights an animal has? What are the rights of a cockroach, or a mosquito? If they are less important than those of a dog or a horse (or, for that matter, a person), how can one ground this difference?

A deontological approach may prove more fruitful. However, the acknowledgement of a general duty not to cause suffering will probably lead to the same problems as the utilitarian approach. A duty to preserve life is a clearer standard. This would, in practice, mean the adoption of ethical veganism, a difficult standard to universalize, but one which may become more attractive as synthetic meat becomes more widely available.