Reply: The interpretation of the words “monotheism” {tawhid} and “polytheism” {shirk} must be in consistency with the Qur’an and the Sunnah of the Prophet (S) because the Glorious Qur’an and the conduct of the Messenger of Allah (S) are the most valuable criteria for distinguishing the truth from falsehood as well as monotheism from polytheism.
Given this, every thought and behavior approved by the awakened and unbiased conscience should be assessed according to the touchstone of the logic of revelation and conduct of the Holy Prophet (S).
The following are solid proofs which the Qur’an and the Sunnah introduced about the permissibility of swearing by other than God:
1. In the eternal verses of the Glorious Qur’an we find examples of swearing by prolific creatures such as the “life of the Prophet”, “soul of man”, “pen” which are manifestations of writing, “sun”, “moon”, “star”, “day and night”, “heaven and earth”, “time”, and “mountains and sea”. We shall cite some of these verses:
a.

"لعمرك إنهم لفي شكرتهم يعمهون."

“By your life, they were bewildered in their drunkenness.”1
b.

﴿ وَالشَّمْسِ وَضُحَاهَا ٭ وَالْقَمَرِ إِذَا تَلَاهَا ٭ وَالنَّهَارِ إِذَا جَلَّاهَا ٭ وَاللَّيْلِ إِذَا يَغْشَاهَا ٭ وَالسَّمَاء وَمَا بَنَاهَا ٭ وَالْأَرْضِ وَمَا طَحَاهَا ٭ وَنَفْسٍ وَمَا سَوَّاهَا ٭ فَأَلْهَمَهَا فُجُورَهَا وَتَقْوَاهَا ﴾

“By the sun and her forenoon splendor, by the moon when he follows her, by the day when it reveals her, by the night when it covers her, by the sky and Him who built it, by the earth and Him who spread it, by the soul and Him who fashioned it, and inspired it with {discernment between} its virtues and vices.”2
c.

وَالنَّجْمِ إِذَا هَوَىٰ

“By the star when it sets.”3
d.

ن وَالْقَلَمِ وَمَا يَسْطُرُونَ

“Nun. By the Pen and what they write.”4
e.

وَالْعَصْرِ {1} إِنَّ الْإِنْسَانَ لَفِي خُسْرٍ {2}

“By Time! Indeed man is at a loss.”5
f.

وَالْفَجْرِ {1} وَلَيَالٍ عَشْرٍ {2}

“By the Dawn, by the ten nights.”6
g.

﴿ وَالطُّورِ ٭ وَكِتَابٍ مَّسْطُورٍ ٭ فِي رَقٍّ مَّنشُورٍ ٭ وَالْبَيْتِ الْمَعْمُورِ ٭ وَالسَّقْفِ الْمَرْفُوعِ ٭ وَالْبَحْرِ الْمَسْجُورِ ﴾

“By the Mount {Sinai}, by the Book inscribed on an unrolled parchment; by the House greatly frequented; by the vault raised high, by the surging sea.”7
Similarly, swearing by the manifestations of the world of creation can also be noticed in Surahs Mursalat (77), an-Nazi‘at (79), al-Buruj (85), at-Tariq (86), al-Balad (90), ad-Duha (93), and at-Tin (95).
There is no doubt that if swearing by other than God is tantamount to polytheism and associating partners to God, the Holy Qur’an which is the charter of monotheism and unity of God would not have resorted to it, and if such a way of swearing is exclusive to God, the verses of the Qur’an would have warned people from doing so lest they should commit mistakes.
2. All Muslims of the world regard the Holy Prophet (S) as their model and consider his conduct and behavior as the criterion for distinguishing right from wrong.
Muslim scholars and compilers of Sahihs and Musnads have mentioned so many instances of the Holy Prophet’s (S) swearing by other than God.
In his Musnad, Ahmad ibn Hanbal, the chief of the Hanbalis, thus narrates from the Messenger of Allah (S):

"فلعمري لأن تتكلم بمعروف تنهى عن منكر خير من أن تسكت."

“By my life! If you would enjoin good and forbid evil, it will be better than keeping silent.”8
In his Sahih, which the Ahl as-Sunnah consider one of the six authentic compilations of hadith, Muslim ibn al-Hajjaj states:

"جاء رجل إلى النبيّ صلى الله عليه وأله وسلم فقال: يا رسول الله أيّ الصدقة أعظم أجراً؟ فقال: أما وأبيك لتنبانه أن تصدق وأنت صحيح شحيح تخشى الفقر وتأمل البقاء."

There came a person to the Prophet (S) and asked the Messenger of Allah: “Which charity is the most rewarding?” He said: “By your father, it is the charity which you give in a state when you are healthy and close-fisted, haunted by fear of poverty, and hoping to live (as rich).”9
How can those who consider a great part of global Muslims as polytheists because they believe that it is permissible to swear by other than God justify this explicit act of the Prophet (S) (i.e., swearing by a person’s father)?
3. Apart from the Book of God and the Sunnah of the Prophet (S), the conduct of the close Companions of the Messenger of Allah (S) also proves the permissibility of swearing by other than God.
In many parts of his sublime speeches, ‘Ali ibn Abi Talib (‘a) swear by his life when he says:

"ولعمري ليضعفن لكم التّيه من بعدي أضعافأً."

“By my life! After me your wandering about shall be multiplied.”10
In another place, he (‘a) says:

"ولعمري لإن لم تنزع غيّك وشقاقك لتعرفنّهم عن قليل يطلبونك."

“By my life, if you do not refrain from hypocrisy, avarice and your rebellious activities, they will soon be known to you.”11
All these traditions and reports show clearly that no ijtihad or exigency can be valid, and no other argument can denigrate the position of God in the Glorious Qur’an, the conduct of the Holy Prophet (S) or the conduct his close companions like the Commander of the Faithful (‘Ali) (‘a) if it accuses them of polytheism and associating partners with God.

Conclusion
From the aggregate of the stated proofs, it is evident that from the perspective of the Book of God, the Sunnah of the Prophet (‘a) and conduct of the believers the legitimacy of swearing by other than God is an indisputable principle, and it has no contradiction with monotheism and the unity of God.
Therefore, if the outward meaning of the traditions opposes that which has been confirmed by decisive proofs, it must be justified and interpreted according to this indisputable principle which is derived from the Qur’an and traditions.
Some people cite an ambiguous tradition which is as follows:

إن رسول الله سمع عمر وهو يقول: وأبي فقال إن الله ينهاكم أن تحلفنا بأبائكم ومن كان حالفاً فليحلف بالله أو يسكت.

Verily, the Messenger of Allah (S) heard ‘Umar swearing by his father. Upon hearing this he said: “Verily, God has prohibited you from swearing by your father, and when one wants to swear one should either swear to God or keep silent.”12
Although this hadith cannot challenge the Qur’anic verses and explicit traditions that consider swearing by other than God as permissible, it must, for the sake of reconciling it and the mentioned verses and tradition, be said that the Prophet’s prohibition of ‘Umar’s swearing by his father and forbidding similar people from swearing by their fathers is attributed to the fact that their fathers were idolaters and polytheists. And an infidel or idol-worshipping person is too unworthy to be an object of swearing.

Notes
1. Surah al-Hijr 15:72.
2. Surah ash-Shams 91:1-8.
3. Surah an-Najm 53:1.
4. Surah al-Qalam 68:1.
5. Surah al-‘Asr 103:1-2.
6. Surah al-Fajr 89:1-2.
7. Surah at-Tur 52:1-6.
8. Musnad Ahmad ibn Hanbal, vol. 5, pp. 224-225, a hadith by Bashir ibn Khasasiyyah as-Sadusi.
9. Sahih Muslim (Egypt), part 3, “Kitab az-Zakah,” “Bab Bayan an Afdal as-Sadaqah, Sadaqah as-Sahih ash-Shahih,” pp. 93-94.
Abdul-Hamid Siddiqui (trans.), Sahih Muslim (English Translation), vol. 1, chap. 29, hadith 2251.
10. Nahj al-Balaghah (Muhammad ‘Abduh), Sermon 161.
11. Nahj al-Balaghah (Muhammad ‘Abduh), Letter 9. For more information about other cases, see Sermons 168, 182 and 187, and Letters 6 and 54.
12. Sunan al-Kubra, vol. 10, p. 29; Sunan an-Nisa’i, vol. 7, pp. 4-5.