Question

why preamble is called the sole of the Indian constitution

Answer 1

'The Preamble' is known as the soul of the Indian Constitution because whenever there arises any doubt in the interpretation of the Constitution the matter is decided in the light of the Preamble.
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Answer 2

The preamble of the constitution of Indiaindicates that the source of constitution is “we the people of India”. It has been also called Political Horoscope of Indian Constitution (by KM Munshi), Soul of the Constitution (by Thakurdas Bhargav) and identity card of the constitution (by NA Palkhiwala).