Some suppose Agur to be asked, as Apollo's oracle was of old, Who was the wisest man? The answer is, He that is sensible of his own ignorance, especially in divine things. Hoc tantum scio, me nihil scire -- All that I know is that I know nothing.
Matthew Henry on Eccl. 1.17-18:
The more we know the more we see of our own ignorance....the more we know the more we see there is to be known...Let us not therefore be driven off from the pursuit of any useful knowledge, but put on patience to break through the sorrow of it; but let us despair of finding true happiness in this knowledge, and expect it only in the knowledge of God and the careful discharge of our duty to him.