Prophecy,
As to the ethics of vegetarianism, it's pretty complicated.
You say that there is no moral defence for eating meat, but that's not right. There are plenty. They share a rejection of cruelty as the only or primary force behind an ethical decision.
But before I even go there, I think it's worth mentioning to everyone that Prophecy is correct about animal sentience, at least as it's defined and discussed in the relevant literature in zoology and animal behaviour.
http://www.mdpi.com/2076-2615/2/4/628/htm
From the abstract of that article: "Animal sentience refers to the ability of animals to experience pleasurable states such as joy, and aversive states such as pain and fear (Broom, D.M. Dis. Aquat. Org. 2007, 75, 99–108)." that seems to be the accepted definition--I may be wrong, but that's what I found.
The literature--I won't cite it all here, everyone can Google--generally accepts that vertebrates (and perhaps some invertebrates, too) experience pain and fear, at least in so far as we can identify these states in their behaviours. From what I've read, "joy" is a tough sell, but "pain and fear" seem to be present in mammals, fish, and even things like crayfish.
In some sense, then, Prophecy is on solid footing. But to move from "cows feel pain and fear" to "therefore, we should not kill and eat them" isn't a simple process, nor is it ethically clear or required. Here are just three examples:
I have a friend, Amy. She is an ardent utilitarian, and her ethics are based on reducing pain and maximising pleasure. For Amy, if the pain a cow feels at slaughter is less than the pleasure experienced by the people who eat it, that eating is justified. This is a simple--maybe too simple--example of utilitarian calculus, but an argument that can certainly be made in defence of eating meat.
I have another friend, Bob. Bob believes that a purposeful god made the world an everything in it, and that he directed man to tame and make use of these resources. For Bob, there is a "hierarchy of being" and a difference in kind between animals and human beings. For him, animals were made for our purposes, and while we shouldn't cause them unnecessary cruelty, raising and slaughtering them in ways that minimise their suffering is perfectly acceptable. This sort of deontological ethic is pretty common, I think.
My third friend, Chris, embraces virtue ethics and has a list of virtues he must practice to lead a good life. Among these virtues, he includes practices like hunting and fishing, without which his life would be diminished. For Chris, these develop critical aspects of his being--self-sufficiency, patience, awareness of nature and our place in it as a predatory species, physical toughness, etc.--allowing him to flourish. That they also cause cruelty to animals, or at least don't eliminate cruelty entirely, is simply not as significant as their role in allowing him to live the good life.
It's not that hard to come up with a much longer list than this. In these cases, the positions I've outlined here are widely held, seriously respected ethical positions applied to this particular question. Not all utilitarians, deontologists, or virtue ethicists would agree with the position I've put forward, but in each case, they could--which is all that matters to demonstrate that the ethics are more complicated than you suggest. I imagine that most people who eat meat, and who've thought about it, agree with some aspects of each of these three positions: they get a great deal of pleasure from eating meat and find that this pleasure is a component of a life they want to lead, but want to minimise the suffering of animals in the meat industry--or perhaps even do away with "industrial" production altogether. Most probably accept, too, that there's an important ethical difference between other animals and human beings. And professional ethicists generally do as well. See this by Martha Nussbaum, for instance:
https://pdfs.semanticscholar.org/39a9/e6a9b2732e13bf19b98a6b0a25952f180530.pdf
My point is only that the inherent cruelty of humane omnivorism is not ethically sufficient to forbid the slaughter and consumption of meat. There are a variety of ethical positions that start from and end in a variety of claims on this issue.
As to the ethics of vegetarianism, it's pretty complicated.
You say that there is no moral defence for eating meat, but that's not right. There are plenty. They share a rejection of cruelty as the only or primary force behind an ethical decision.
But before I even go there, I think it's worth mentioning to everyone that Prophecy is correct about animal sentience, at least as it's defined and discussed in the relevant literature in zoology and animal behaviour.
http://www.mdpi.com/2076-2615/2/4/628/htm
From the abstract of that article: "Animal sentience refers to the ability of animals to experience pleasurable states such as joy, and aversive states such as pain and fear (Broom, D.M. Dis. Aquat. Org. 2007, 75, 99–108)." that seems to be the accepted definition--I may be wrong, but that's what I found.
The literature--I won't cite it all here, everyone can Google--generally accepts that vertebrates (and perhaps some invertebrates, too) experience pain and fear, at least in so far as we can identify these states in their behaviours. From what I've read, "joy" is a tough sell, but "pain and fear" seem to be present in mammals, fish, and even things like crayfish.
In some sense, then, Prophecy is on solid footing. But to move from "cows feel pain and fear" to "therefore, we should not kill and eat them" isn't a simple process, nor is it ethically clear or required. Here are just three examples:
I have a friend, Amy. She is an ardent utilitarian, and her ethics are based on reducing pain and maximising pleasure. For Amy, if the pain a cow feels at slaughter is less than the pleasure experienced by the people who eat it, that eating is justified. This is a simple--maybe too simple--example of utilitarian calculus, but an argument that can certainly be made in defence of eating meat.
I have another friend, Bob. Bob believes that a purposeful god made the world an everything in it, and that he directed man to tame and make use of these resources. For Bob, there is a "hierarchy of being" and a difference in kind between animals and human beings. For him, animals were made for our purposes, and while we shouldn't cause them unnecessary cruelty, raising and slaughtering them in ways that minimise their suffering is perfectly acceptable. This sort of deontological ethic is pretty common, I think.
My third friend, Chris, embraces virtue ethics and has a list of virtues he must practice to lead a good life. Among these virtues, he includes practices like hunting and fishing, without which his life would be diminished. For Chris, these develop critical aspects of his being--self-sufficiency, patience, awareness of nature and our place in it as a predatory species, physical toughness, etc.--allowing him to flourish. That they also cause cruelty to animals, or at least don't eliminate cruelty entirely, is simply not as significant as their role in allowing him to live the good life.
It's not that hard to come up with a much longer list than this. In these cases, the positions I've outlined here are widely held, seriously respected ethical positions applied to this particular question. Not all utilitarians, deontologists, or virtue ethicists would agree with the position I've put forward, but in each case, they could--which is all that matters to demonstrate that the ethics are more complicated than you suggest. I imagine that most people who eat meat, and who've thought about it, agree with some aspects of each of these three positions: they get a great deal of pleasure from eating meat and find that this pleasure is a component of a life they want to lead, but want to minimise the suffering of animals in the meat industry--or perhaps even do away with "industrial" production altogether. Most probably accept, too, that there's an important ethical difference between other animals and human beings. And professional ethicists generally do as well. See this by Martha Nussbaum, for instance:
https://pdfs.semanticscholar.org/39a9/e6a9b2732e13bf19b98a6b0a25952f180530.pdf
My point is only that the inherent cruelty of humane omnivorism is not ethically sufficient to forbid the slaughter and consumption of meat. There are a variety of ethical positions that start from and end in a variety of claims on this issue.
