This book is the first monographic study of prakrti. It traces the history of the word prakrti, the word chosen by the modern North Indian languages to translate the English word 'nature, ' through the texts of the Hindu, Buddhist, and Jain systems of religious thought. Prakrti is one of the central metaphysical principles in the religious traditions of Hinduism, especially in the very influential Samkhya and Yoga traditions. The second part of the book gives a systematic analysis of this important principle in the Proto-Samkhya, Samkhya, and Samkhya-Yoga texts. Prakrti in the Samkhya and Yoga systems is the ultimate material principle and thus the substratum (dharmin) from which manifest, in the presence of the self (purusa), the gross and subtle bodies including the mental organs of all living beings from the leaf of grass to the powerful gods. Everything that becomes manifest is held together as causes and effects within this immense power. This ultimate material principle has also been an object of religious realization. This book investigates a religious experience called 'merging with prakrti' (prakrtilaya), which has been neglected in previous studies, but is described in the Samkhya and Yoga texts and constitutes an important aspect of the religious understanding of the material principle. Finally, this book shows the important implications for issues of interspecies or environmental ethics of the understanding of the material principle in the Samkhya and Yoga systems of religious thought.