Num 8:7

“Hairs of the flesh mean whatever human corruption is left. Hairs of the flesh are the thoughts of the old life, which we so expel from our minds that no grief at their loss fatigues us. Levite means "one taken up." So all Levites should shave the hairs of the flesh. For he who is taken up into divine service should appear before the eyes of God cleansed of all carnal thoughts. His mind should not bring forth illicit thoughts and deform the beautiful shape of his soul with unruly hair. But as much as the virtue of holy conversation draws a man up, as we said, he was still born into the old life, and he bears it with him. Thus the hairs of the Levites are to be shaved off, not pulled out. For when hairs have been shaved off the flesh the roots remain, and the hairs grow and are shaved off again. Vain thoughts should be cut off with great effort, but they can never be entirely rooted out. For the flesh always begets what is vain, and the spirit cuts it back with the knife of watchful concern. We see this happening in us more subtly when we reach the heights of contemplation. Exposition of the Old and New Testament, Numbers”