Surely, I’d thought, science can defend the obvious, that slaughterhouse carnage is wrong in a way that harvesting a field of lettuces or, say, mowing the lawn is not. But instead, it began to seem that formulating a truly rational rationale for not eating animals, at least while consuming all sorts of other organisms, was difficult, maybe even impossible.

Before you hit “send” on your hate mail, let me say this. Different people have different reasons for the choices they make about what to kill or have killed for them to eat. Perhaps there isn’t any choice more personal or less subject to rationality or the judgment of others. It’s just that as far as I was concerned, if eating a tofu dog was as much a crime against life as eating bratwurst, then pass the bratwurst, please.

So what really are the differences between animals and plants? There are plenty. The cells of plants, and not animals, for example, harbor chloroplasts, tiny green organelles that can turn the energy of light into sugar. Almost none of these differences, however, seem to matter to any of us trying to figure out what to eat.

The differences that do seem to matter are things like the fact that plants don’t have nerves or brains. They cannot, we therefore conclude, feel pain. In other words, the differences that matter are those that prove that plants do not suffer as we do. Here the lack of a face on plants becomes important, too, faces being requisite to humans as proof not only that one is dealing with an actual individual being, but that it is an individual capable of suffering.

Animals, on the other hand — and not just close evolutionary relations like chimps and gorillas, but species further afield, mammals like cows and pigs — can experience what pretty much anyone would agree is pain and suffering. If attacked, these animals will look agonized, scream, struggle and run as fast as they can. Obviously, if we don’t kill any of these animals to eat them, all that suffering is avoided.