As-Salaam alaikum,
The Apostle of Allah, Sallallahu alaihi Wasallam, said: "He who knows himself knows his Lord" if he knows himself as perishable, he knows Allah as Everlasting, or if he knows himself as humble, he knows Allah as Almighty, or if he knows himself as a servant, he knows Allah as the Lord.

Therefore one who does not know himself is debarred from knowledge of all things..

Many Regards.

Like to read/see more on this?
Refer to the work titled "Kashful-Mahjub", by
Maulana Ali Ibn Usman al-Hujwiri.