They arrived at this idea by considering the number five, the
number of the senses, as the mystical opponent of the visible and
sensible universe-- ta aistheta, as distinguished from ta noita.
Origen lays down the rule in express terms. '"The number five,"'
he says, '"frequently, nay almost always, is taken for the five
senses."' In another passage, Keble deals with an even more
recondite question. He quotes the teaching of St. Barnabas that
'Abraham, who first gave men circumcision, did thereby perform a
spiritual and typical action, looking forward to the Son'. St.
Barnabas's argument is as follows: Abraham circumcised of his
house men to the number Of 318. Why 318? Observe first the 18,
then the300. Of the two letters which stand for 18, 10 is
represented by 1, 8 by H. 'Thou hast here,' says St. Barnabas,
'the word of Jesus.' As for the 300, 'the Cross is represented by
Tau, and the letter Tau represents that number'.

Unfortunately, however, St. Barnabas's premise was of doubtful
validity, as theRev. Mr. Maitland pointed out, in a pamphlet
impugning the conclusions of the Tract. 'The simple fact is,' he
wrote, 'that when Abraham pursued Chedorlaomer "he armed his
trained servants, BORN IN HIS OWN HOUSE, three hundred and
eighteen". When, more than thirteen (according to the common
chronology, fifteen) years after, he circumcised "all the men of
his house, BORN IN THE HOUSE, AND BOUGHT WITH MONEY OF THE
STRANGER", and, in fact, every male who was as much as eight days
old, we are not told what the number amounted to. Shall we
suppose (just for the sake of the interpretation) that Abraham's
family had so dwindled in the interval as that now all the males
of his household, trained men, slaves, and children, equalled
only and exactly the number of his warriors fifteen years