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Answer
Hindu Dharma has no single founder. It is the cumulative result of the knowledge, austerity (sadhana), inner discovery, and lived experience of countless rishis (male sages) and rishikas (female sages).
Science, too, has no single founder. Science is the body of knowledge developed and accumulated through the experiments, research, and experiences of many scientists across centuries. In the same way, Hindu Dharma also has no one founder.
The oldest scripture of Hindu Dharma is the Rigveda. Therefore the rishis and rishikas of the Rigveda may be regarded as the earliest founders of Hindu Dharma.
If there is no single founder, there can hardly be a single book — and that is precisely the next question in this lesson.