Zeno: The philosopher David Hume pointed out that one can not logically deduce an "ought" from an "is." There is nothing that we "ought" to do. Morality is not an element in the periodic table, nor is it an objective and inalterable property of the universe. Our morality will depend on our desires and circumstances. If we wish to live and be happy, there are certain things that we need to do, but there is no "ought." There is no universal cosmic code of justice that we can hold up as our standard for comparison.
It is not obvious that we "ought" to do anything. Oughts are metaphysical baggage from an earlier time. The concept does not bear close scrutiny any more than the traditional concept of the soul. There are however things we "need" to do in order to be happy and healthy. If a sound argument can be made that veganism can make us happier and healthier, then we might say that it is reasonable to adopt it if we wish to be happy and healthy, though there is the further problem that different people may of course mean different things by "healthy" and "happy."
Plato: Sam Harris says in the Moral Landscape that morally good actions are ones that increase the well-being of conscious creatures.
Zeno: Yes, he takes that as an axiom. But people may or may not agree with this choice of axiom. If we agree, then his argument is valid. Why should we however prefer the well-being of all conscious creatures? And how shall we define "conscious"? This is murky territory.
I for my part agree with Sam; this is my preference, but I am not sure that he has demonstrated that there is anything we "ought" to do, only what we need to do in order to be consistent with this particular choice of axiom.
Plato: I think he says science can tell us what we ought to do in order to move towards the well-being of all creatures.
Zeno: It seems that he is using "ought" to mean what is "necessary." If we first accept that our objective is the well-being of all conscious creatures, then he makes a good argument. I am just not sure how one could logically justify taking the well-being of all conscious creatures as an axiom. It seems that this can only ever be a preference, and not all people have the same preference. Though I share Sam's preference, my neighbor might prefer the well-being of only aardvarks for example.
We observe that most people value their own well-being and that of those closest to them, but if we wish for people to adopt a principle such as veganism, can we make the case that people will be healthier, happier etc. if they adopt it?
Science can not tell us to prefer chocolate to vanilla, life to death, health to sickness. We have basic desires, and science can show us how to be consistent with those desires, or how to achieve our targets. People may disagree about the definitions of happiness, health, etc. Most of us agree that it is better to be alive than dead, fed than starved, clothed than naked, but this is not a universal. If we agree on our preferences, then we may proceed from that basis to determine whether a given principle, in this case veganism, is consistent with that set of preferences.
Plato: But what else could be more morally good than the well being of conscious creatures? If we accept this then surely science can direct our choices.
Zeno: That is a good question. What if my neighbor believes that morality is defined as what is good for our species? Yes, I agree that once we accept an axiom, science can indeed inform our decisions.
Plato: I think the axiom would still hold because it is self-evident i.e. the wellbeing for all conscious creatures has to be better than the wellbeing for just some conscious creatures? True your neighbour probably wouldn't accept it but that shouldn't change our thinking any more than another neighbour who favours his own race, or sex.
Zeno: It is not self-evident. If it were, everyone would agree on the axiom.
Evolution has meant that we tend to favor our own survival over that of other species. One might argue however that the continuation of other species is relevant to our own well-being.
The cold, hard question is why should one care more about a rabbit than a radish? Where do we draw the line? Why exactly are lions more important than bacteria? And even if we do agree that consciousness is relevant, how do we determine what is sufficiently conscious to deserve our protection? Consciousness lies on a continuum. Where we draw the line will be arbitrary. If you say that rabbits are more deserving of our protection than ants, you will have to justify that.
But why have we decided that some types of life are more worthy of our consideration than others? This is a matter of preference. I hope perhaps that people will share my preference, but I can not pretend that there is some universal moral standard that obligates people to treat rabbits differently from radishes or bacteria.
Consider cultures where it is acceptable to eat dogs. Those people who choose to eat dogs apparently feel little to no empathy for their food of choice. I find their choice disturbing. It certainly seems self-evident to me that one should not harm dogs, but it is not self-evident to them,
One must make a sound argument to support the notion that we should consider the well-being of all "conscious" (however that is determined) life. Self-evident notions on the other hand require no explanation as they are intuitive to any given observer. That is what self-evident means.
Plato: I believe that the wellbeing for all conscious creatures is morally the best possible world to work towards. This is self-evident to me but I understand your objection that it should be self-evident to everybody or otherwise it should not be called self-evident. But by this definition surely we cannot call anything self-evident because we will never know if everyone finds it self-evident. Perhaps we should call it true instead. If we know something is true, such as evolution, it makes no difference to the truth claim if half the world doesn't believe it to be true.
Why should one care more about a rabbit than a radish? Because a rabbit is a conscious sentient creature. Why exactly are Lions more important than bacteria? Because Lions are conscious sentient creatures.
Where we draw the line on a continuum matters little in the real world of animal suffering but I understand it is a matter of importance in philosophy. It is possible that rabbits aren't more important than ants but we can easily stop our contribution to the suffering of rabbits by being vegan.