So if an atheist holds that (a) there are no souls, and (b) killing is bad - then the only logical course of action for her is to be vegetarian. She can eat plant products, because eating plant products doesn't necessarily require killing the entire plant. But if she is to respect the value of life, she cannot eat any meat since killing a living creature would be unavoidable.
If your atheist vegan drives in the summer, insects (which probably are not “robots”) will find their deaths in her car's grill. And this is not an unforeseeable accident, it is guaranteed to happen. So she “should” (the vague word used in moral realist discourse) not drive.
But as J. L. Mackie noticed amusedly, “anyone who really means business uses ‘must’ [instead of should]”.
Do you think she must not drive with her car in the summer? I doubt it.
So, there is no good way to judge partial compliance. You could also claim that everyone who does not live like a Jain nun or monk (sans celibacy) is not a moral person, but this would border on the absurd.
Is this reasoning valid? If one claims to be both a materialist atheist and a moral person, then the only choices they have are being vegetarian or eating carrion?
Why “if”? Why can carnivorous soul-believers claim to be moral persons?
Do you think that the Aztec priests were moral people because it followed from their religion that human sacrifices were good, pious acts?
Well, if you're already so deep into moral relativism, some inconsistency is the least you should worry about.
She must not drive if and only if insects are sentient enough to receive moral consideration. The link is an indication that it shouldn't be dismissed. As Singer argues, with vertebrate animals we can be sure that they can suffer.
Why not give insects the benefit of the doubt?
One could also argue that she can't avoid driving, but can avoid eating meat, and add that "Ought implies can".
Many people don't have a driver's license or a car and they still manage to live their life. Also, much driving is connected to definitely non-essential purposes like leisure.
But even if we dismiss the driving argument, we can quickly construct another:
In modern plant agriculture, there is no way to avoid killing vertebrates like mice, lizards, snakes, rabbits, etc., this is caused unintentionally by ploughing and harvesting and intentionally by vertebrate pest control.
Since human survival depends on plant agriculture, we “can't” just stop doing it.
But... this doesn't excuse that most vegans do not live by a principle of truly “maximal harm avoidance”. Let's say:
- Alice eats a rich omnivore diet. She especially loves steaks, burgers and curry chicken.
- Betty eats a rich vegan diet. She likes tofu-vegetable lasagna, vegan pasta, vegan pizza, vegan sweets, etc. and also drinks coffee.
- Carol uses a vegan diet that causes the least animal suffering and provides her with enough calories and essential nutrients. This means Carol exclusively
- eats bland dishes made of a small variety of highly efficient crops with minimal impact on animals
- takes mineral/vitamin-supplements
- drinks water.
Betty obviously causes massively less animal deaths and suffering than Alice, but Carol still causes noticeably less animal deaths and suffering than Betty.
“Ought” we to live like Carol? And if we don't because we just can't be bothered to, what then? Do we stop to be a moral person? Is Betty immoral? And if she isn't, why isn't Alice, too? Don't you think that Alice, Betty and Carol are all pretty far removed from a person which we judge to be truly immoral like an unrepentant murderer?