Somewhat arbitrary assignment based on self-assessment. When I see a person in danger, say, I go, "huh, it'd be a shame if he doesn't get off those railroad tracks in time. I feel X amount of anxiety over the possibility of his loss." And then when I see a deer picking its way across eight lanes of traffic, I feel pretty much X amount of anxiety as well. Then, using mental exercises, I can imagine what I'd feel like if a housecat were to be eaten by a great white shark, if a bunny were to be struck by lightning, or if a player piano were to fall on a bengal tiger. I feel pretty similar amounts of death-anxiety in each instance. We all sort of follow our conscience, but we don't really get to choose what it tells us. I'm just reporting my own findings to you. For me, I'm pretty sure the line is drawn at the edge of the mammal class.On the other hand, say there's a spider, and some demented little kid finds it, tortures it, pulls off its legs one at a time, visits all sorts of chemical horrors on it, calls it names and demeans it in a way that you just know makes the spider feel really, really, bad, and then squishes it with a brick. This causes me Y amount of anxiety. Here, Y = 0. In fact, I feel delight, as spiders are ####ing creepy and I want them all gone.