Is the designation as "matter" vs. "antimatter" arbitrary? I think this should be a very elementary question, but I haven't seen it addressed anywhere before. (but pardon me if it has been, and I just missed it.) I know that it is arbitrary to designate the electron as a matter particle and the positron as antimatter, but once this designation has been made, is there any connection that forces us to designate the up and down quarks as matter as well? For that matter (pun unintended, honestly!), does the designation of the up quark as "matter" force us to designate the down quark the same way? I think I'm asking if there is any significance to the designation as "matter" other than between particles that have otherwise identical quantum numbers?