Pesachim 7b: A 'chain' [of verses] teaches that a Ner is appropriate [to check for Chametz, to avoid "Lo Yimatzei"]. We learn Metzi'ah from Chipus, and Chipus from Neros. It says "va'Ychapes ba'Gadol ha'Chel... va'Yimatzei," and "va'Achapes Es Yerushalayim ba'Neros," and "Ner Hashem Nishmas Adam Chofes Kol Chadrei Veten."

Rashi: Man's Neshamah testifies about him in judgment.

R. Yonah #1: A Ner causes that one can see what is in the house. So the Neshamah causes Hashem's Hashgachah in a person. Hashem tests and investigates his heart and kidneys; the Neshamah is from above. Therefore man must rule over his deeds also in his thoughts. He is proper to be punished for sin more than other living beings.

R. Yonah #2: Ner Hashem is a lamp that Hashem sends. "V'Ner Elokim Terem Yichbeh" (Shmuel I, 3:3) is the Ner that Hashem commanded to arrange it. "K'Harerei Kel" (Tehilim 36:7) are the mountains that He formed. Man's Neshamah is Hashem's lamp in the body; it is like the supreme light in the world, to illuminate in man and make him wise. Therefore, he is commanded about his thoughts.

Malbim: Hashem does not need a wheel to afflict Resha'im to force them to admit to clarify who sinned. Man's Neshamah is Hashem's Ner to illuminate to enable Him to know what is hidden in darkness ¹ . It searches all internal 'rooms' to know all hidden matters; it testifies about man.

Erech Apayim (2:1, citing Tzaharayim Pinchas): When one engages in Torah with fervor or prays with Kavanah, his limbs move stragely, for the fire of Torah is ignited in him.