Proverbs 14:10

14:10 The heart knows its own bitterness,

and with its joy no one else can share.

Proverbs 20:5

20:5 Counsel in a person’s heart is like deep water,

but an understanding person draws it out.

Proverbs 20:27

20:27 The human spirit is like the lamp of the Lord,

searching all his innermost parts.

Jeremiah 17:9

17:9 The human mind is more deceitful than anything else.

It is incurably bad. Who can understand it?