Al-Biruni did not find caste system unique to India because according to him in ancient Persia too- four social categories were recognised. However, at the same time, he also pointed out that no such divisions existed in Islam. Though Barni approved the Indian caste system, he critised the notion of 'pollution' or untouchability. He remarked that everything which falls into a state of impurity strives and succeeds in regaining its original condition of purity. For example, the Sun cleans the air and salt in ocean prevents the water from becoming polluted. Thus according to Barni, concept of untouchability in the caste system was contrary to the laws of the nature.
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