I'm not here going to examine either the arguments Singer gives for his view or look in more detail right now at the dispute concerning vegetarianism. What I would suggest, though, is that the dispute concerning it is akin to that which exists with regard to abortion. Those who proclaim the moral impermissibility of abortion allege that the foetus should be treated as a "person" in the sense that termination of its existence is equivalent to murder. This argument turns on the claim that the foetus is sufficiently akin to persons of other sorts as to merit treatment that would be correct for the latter.
In both the cases of vegetarianism and abortion there are central disagreements concerning where the area of moral concern correctly gets drawn and what is at stake in addressing something as equivalent to a "person". In a basic sense the disputants are divided over this question and thus draw the boundaries of the moral terrain differently. Because this is so there is something intractable about the debate. This doesn't entail that people are incapable of changing their view in either case. In both examples I have given we do find changes of view take place though it is also true, in both cases, that this is often due to emotive appeals having an effect rather than because practical reason has come to have a newly described and more apt shape.
The point of elaborating that this is the type of disagreement in these cases is to try to defuse the sense of scepticism about morality that often arises from consideration of such hard cases. It is because dispute in these hard cases is difficult and often unproductive that many come to the view that morality is not susceptible of rational argument at all and that such disputes are either matters of opinion (in much the way as is thought to be true of matters of taste) or that it is only by reference to emotional appeal that anything can be altered in a dispute.
To adopt the view that morality is, as taste is said to be, something particular and subjective, is to give up on practical reason just as surely as adoption of an emotivist view entails doing. But both ignore the wide range of moral agreement that exists. There is little dispute concerning the wrongness of murder even if there are border-line arguments concerning the applicability of the category (as we saw with the case of abortion). This indicates that, when it comes to the central issues of ethics, there is a general agreement, which we could follow theorists of the 19th and 20th century and refer to as "common sense morality". This "common sense morality" is something that requires to be related to grounds of justification precisely because of the temptations of moral scepticism and it is these grounds of justification, grounds that we can term "meta-ethical" that form the subject matter of much ethics. Whilst those engaged in "applied" or "practical" ethics look at the areas of actual wide disagreement and try to work through its basis the "meta-ethical" focus on the justification of the generally agreed core of morality is just as significant and is the true ground of resistance to moral scepticism.