The present work is designed to survey the evolution of philosophical thought in the Vedic and post-Vedic periods preceding the rise of Jainism and Buddhism. The author has traced up the development of early Indian philosophy on divergent lines on the basis of the Rgveda, Atharvaveda, Aranyakas, the older Upanisads and the allied literature. The work is divided into four parts: each part is divided into several chapters. Part I deals with Vedic philosophy; Part II with post-Vedic philosophy; Part III with the philosophy of transitional period before Mahavira and Buddha and Part IV with the philosophy of Mahavira. The author has exploited the original Indian sources and in defiance of several scholiasts has proved that the process of early Indian thought evolution is neither unscientific nor unsystematic. The work throws abundant light upon a very obscure and highly important period of Indian thought. It is also a very useful study for ascertaining the immediate background of Buddhistic philosophy.