I think for most vegetarians it's the capacity for suffering as we know it from ourselves.<p>Of course plants have defense routines that harvesting might trigger, but only for stones I'm rather sure they don't care if I smash them or not and I can't eat those.<p>Everybody draws some line somewhere, for most this is not eating humans and hopefully not just to avoid prision or the danger of beeing eaten themselves. For some it's hunting themselfes so there is no unnecessary suffering, for others it's another compromise between comfort and ethics.<p>In any way I think most of these thoguht-through positions make sense and they all aim to reduce suffering, which I see as a very good thing.