{If I then} (ei oun eg(935c)). Argumentative sense of oun
(therefore). Condition of first class, assumed to be true, with
first aorist active indicative of 
ipt(935c), "If I, being what I
am, washed your feet" (as I did). {Ye also ought} (kai humeis
opheilete). The obligation rests on you _a fortiori_. Present
active indicative of the old verb opheil(935c), to owe a debt ( Mt
18:30 ). The mutual obligation is to do this or any other needed
service. The widows who washed the saints' feet in  1Ti 5:10  did
it "as an incident-of their hospitable ministrations" (Bernard).
Up to 1731 the Lord High Almoner in England washed the feet of
poor saints (_pedilavium_) on Thursday before Easter, a custom
that arose in the fourth century, and one still practised by the
Pope of Rome.