modern icelandic version would be 'þekktu sjálfan þig' which can be said
'þekk þú sjálfan þig'
in modern icelandic kenna means to teach. it had a broader meaning in old
norse, true, but it could mean to teach, to know apart, or to recognize...
þekkja meant to recognize too. i think you could, as such, use either, but
judging from Lexicon Poeticum, i'd say using the verb þekkja would be less
easy to misunderstand... and for the record, vita just isn't as wide a word
as is the english know. know is a very wide word, it means a lot of things
which we differentiate between. vita only means to know, as in the sense of
having knowlege of or about a subject.
i'd guess 'þekk sjálfa(n ) þik' or 'kenn sjálfa(n) þik', but neither looks
all that good to me... (of course, that sentence doesn't look good in any
language other than greek, but that may just be my opinion)
i'd like to see what our lord and master haukur says about this...