The major obstacles to tackling this essay are the impenetrable prose and unnecessarily complex vocabulary. The armoury of the educated to defend their assumed intellectual superiority.
From my lowly standpoint it would appear to me that Greenberg is suggesting or rather stating that to legitimise or justify art we must criticise it but before we can do that we must understand what it is and what it is endeavouring to achieve.
He implies that the only medium capable of representing modernism is painting as painting is the only medium that can divorce itself from representation and what has gone before by virtue of its flatness; the essence of Modernism.
http://www.sharecom.ca/greenberg/modernism.html (Accessed 20.10.08)
http://homepage.newschool.edu/~quigleyt/vcs/mp_sum.html (Accessed 20.10.08)