Consciousness and the Unseen give us dualistic principles for knowledge as well. Bodily pains contrasted with physical pleasures form another duality linked to Heaven and Hell in our minds. So good and evil form another pair (4:xi) as we find them throughout God's creation; they both come ultimately from God, but are not coequal (5:xviii). We need to turn evil into good (7: xii) since they all derive from God (5: xiii). God may have permitted the presence of evil, but He did not command its existence.
"Where is night when the day arrives?" asked the Prophet rhetorically when he was questioned on this matter of good and evil. We also hear:
The blind and the sighted are not equal
nor are darkness and light
nor a shady nook and a heatwave.
The living and the dead are not alike,
God lets anyone He wishes listen, while you
will not make those in their graves hear.
(Originator 35:1922).
If there were other gods
in either [Heaven or Earth]
besides God [Alone],
they would both dissolve in chaos.
(Prophets 21:22)
This concern for twin principles led to the socalled "Persian error" which the Albigensians practised in southern France until one of the first crusades wiped them out in the twelfth century, the onset of the great pogroms of Europe, as that continent flexed its muscles to practise genocide. Manicheism, as it is called formally, had been brought to western Europe by Roman soldiers before the fall of their empire, if it had not been a remnant from ancient IndoEuropean folk religion, and a sister of Celtic Druidism. The Bogomils in the Balkans formed another dualistic cult like it; in the fourteenth century they welcomed the Turks and became Muslims to escape the political and religious tyranny of both Rome and Byzantium as represented in the Catholic and Greek Orthodox churches respectively. In this case Islam benefited from the industrious Slavs and Albanians who joined Islam, and who contributed statesmen and architects to the Ottoman empire.
In Islam the sin of dualism is part of Association or shirk, and thus is unpardonable (4:48, 116). This "Persian error" considers the presence of evil to be necessary, as are the other pairs like Light and Darkness, Night and Daylight: allayl wa alnahar of 2:164 etc. There has been overmuch study of Jewish and Christian sources for the antecedents of Islam, but little of Manicheism and Zarathustrianism.
How to be both artistic and correct with the dual in Arabic becomes a syntactic problem when it comes to translation into English. A dual of sorts is found in the English words "either" and "both", but English has no adequate inflections for expressions like "both of them", "the two" etc., except with parents and other married couples, such as with Adam and Eve (7:ii). However 'both' is generally the best distinguishing word in English, with verbs and nouns as well. Its use occurs with Moses and Aaron in 20:ii ("they both said"); also 10:viii and 26:ii, and with David and Solomon (21:vi, 27:ii