Anthony asks whether this scholastic distinction between accidental and essential predication makes any sense. I think it does. Ockham says, in Chapter 25 of the magnificient Summa Logicae that “An accident is what is present or absent without the corruption of the subject”. So ‘Socrates is sitting’ predicates an accident of Socrates, for sitting can ‘be present’ or ‘be absent’ in Socrates without Socrates being corrupted, i.e. ceasing to exist. By contrast ‘Socrates is a man’ is essential predication. If ‘man’ ceases to ‘be present’ in Socrates, then Socrates ceases to exist. Socrates is nothing if he is not a man. But he is still something if he is not sitting.
The medieval philosophers fretted considerably about whether, pointing to dead Socrates, we are pointing to a man or not. But we can leave that problem for now.