Radak (2): It is Zera Yakov, who seek Hashem and were commanded to be Nekiyei Chapayim u'Varei Levav. And so they were often - in the days of Yehoshua and the Zekenim after him, in the days of the Shoftim, while the Shofet lived, in Shmuel's lifetime "va'Yinahu Kol Beis Yisrael Acharei Hashem" (Shmuel I, 7:2), and in the days of David, Shlomo and the good kings. No other nation served Hashem together.

Rashi: He did not swear in My name and Nefesh in vain. We find Nefesh regarding Shevu'ah - "Nishba Hashem Tzevakos b'Nafsho" (Yirmeyah 51:14).

Radak: It is pronounced Nafshi, for Hashem said so - "Lo Sisa Es Shem Hashem Elokecha la'Shav" (Shemos 20:7). It is written Nafsho, for even the Nefesh of the one who swears, he should not take it in vain (i.e. 'I swear by my life' - PF).

Radak: He swears only truthfully and righteously. It is a Mitzvah for Yir'ei Hashem to swear - "Es Hashem Elokecha Tira Oso Sa'avod u'Vo Sidbak uvi'Shmo Tishave'a" (Devarim 10:20) - if you have all these Midos, then you may swear in His name.

Malbim: In the womb, a lamp is above him. When he is born, an oath is put on him - 'be a Tzadik, and not a Rasha' (Nidah 30b). The lamp is the Divine Neshamah - "Ner Hashem Nishmas Adam" (Mishlei 20:27). The Shevu'ah is the ethical and good Midos planted in man. He did not transgress this Shevu'ah and his sense of ethics that forces him to do what is good and straight.

Radak: They correspond to action, thought and speech. Man should be complete in all of them - clean deeds, a clear heart and trustworthy in his words.

Malbim: His deeds are clean, and the thoughts and mindset of his heart are clear. Hashem gave to man preparations for perfection and happiness - (a) his Divine Nefesh, which is called Nafshi, for it is a part of Hashem. It will lead him to Har Hashem if he will not take it to vain and folly to break its perfection; (b) the powers that Hashem put in man, most lean for the good, e.g. Chachmah and ethical understanding.