We, being closer to other beings in the kingdom Animalia, naturally find it easier to understand and to extend our compassion to the rest of the kingdom. We can easily recognise analogous behaviour and expression in other mammals, such as fear, happiness, sadness, anger, boredom, etc..

We struggle more with recognising emotions — clear evidence of consciousness and by extension, self-interest — in birds, fish, and insects.

We don’t recognize — and most of us don’t even want to think about — consciousness in beings within the other four kingdoms: Plantae, Fungi, Protoctista and Monera. We find it much more difficult to understand and feel compassion for these beings as they are so unlike ourselves.

I do think we need to draw the line somewhere, regardless of whether we can be sure about the self-interest of other beings, and I do agree that a vegan lifestyle harms much fewer beings, both in the immediate sense of being destroyed for our consumption, and in terms of harm to ecological systems. In short, while I think caution is in order when making assumptions about other beings and their self-interest, I do think that veganism is a better, less harmful, choice.