Truthseeker

Help!  I am in over my ‘kephale’ on this one!  In reading D.A. Carson’s Exegetical Fallacies, he says those who translate kephale as source are mistaken as that meaning only occurs in classical Greek and they should be looking at the Hellenistic (koine or common?) definition which is the lingo the N.T. was written in, which would not reveal ‘source’ as a definition for this time period but would yield ‘leader’ as a definition.  My comp spouse pointed this out to me in our ongoing debate so I need to hunt this one down.  Any experts out there on this one?  I posted this here in the headship section even though this is an older post because others like myself might look under ‘headship’ for this issue.