Mesilas Yesharim (Perek 6): I contemplated and saw the great evil - it is like venom that spreads slowly, and it is not recognized until death.

Malbim: Chizayon is vision (understanding) of the heart and Ru'ach. Re'iyah is sight of the eyes. He put the former to his heart; the latter, he saw and took Musar. The Nimshal is the field and vineyard of the Nefesh. The Nefesh has potential to yield fruits of Chachmah and talent, if one plows valleys and removes thorns, and seeds Zera Hashem, like I explained in Yeshayah (28:24). The field is a Mashal for Mitzvos Aseh that one must do in action. The work that one must do in the field is investigation to gather and enter. An Atzel does not do these, due to laziness. The vineyard is a Mashal for one who already planted vines of Chachmah; he must guard Mitzvas Lo Sa'aseh, and guard his learning lest he forget it. This is easy. One who slackens, it is only due to Chasar Lev. He has no power that rules over his Ru'ach. Desire breaches to transgress Lavim and abandon Torah. His field and vineyard became full of thorns. His Nefesh is ruined via the thorns of the Yetzer, and it does not produce its fruit. From this I took Musar to afflict the Nefesh with the ropes of Musar, lest it leave its role.