I have a theory (plausible?)about the origin of the phrase "to lose one's marbles"
I suggest that it's the opposite of "knowing your marbles" This was a phrase used to imply a knowledgeable person ewho could identify the source of all the pieces of Italian marble which were inlaid into expensive tables of the
18th(?) century. You can see them at museums... e.g. the Gilbert collection in London. Any comments??