The only philosophy and principle that is logical and methodical is the philosophy of non-duality. The Eastern masters have explained the infinite & diverse nature of the universe and her manifestations through intricate observations and meticulous analysis over the last 10,000 years.
The phrase that best describes the essence of non-dual universe is “like a sphere whose center is everywhere & circumference is no where“.
- Kybalion: The oldest known book that propounds the principles of non-duality in 7 principles is the Kybalion, the principles being Mentalism, Correspondence, Vibration, Polarity, Rhythm, Cause and Effect, and Gender. Kybalion is authored by the “Three Initiates” who have remained anonymous. You can read the text here.
- The Emerald Tablet of Hermes: Very similar to the Kybalion in essence, the Emerald Tablets is a work on natural philosophy of creation & alchemy, following the principles of which one can be transformed into the one that is the creator of the universe. You can download a pdf copy here.
- Tao Te Ching: This is an ancient Chinese text with 81 verses and explains the nature of life & the universe. The text is hard to translate into Western languages and various translators have used their grace & imagination in describing the book & the texts, but largely they all speak of the oneness of the existence and the reasons behind this apparent diversity we see around us. One free pdf version is available here.
- Ashtavakra Gita: This ancient Indian text is a dialog between Ashtavakra & the king of Mithila, Janaka, who is about to realize the Truth through self inquiry. Of the numerous Gitas on the Indian philosophy the Ashtavakra Gita is at the acme of non-dual teaching and drives straight to the point of the oneness of our existence.
- The Gospel of Sri Ramakrishna: Unlike the others, this is a document showing the day in a life of Sri Ramakrishna, who has lived the Vedas & the Vedantas every day of his life. Unless someone read this he would not be able to understand the true execution of the messages contained in these non-duality texts. Sri Ramakrishna has shown through his own life how to be the non-dualist in spirit & in daily life.
Last but not the least, Sri Nisargadatta Maharaj expounded his experiences from being an ordinary person to achieving the realized non-dual state in his numerous lectures many of which were later translated by Western authors, the most authentic one being by Maurice Frydman. The one that hits the point directly is a conversation translated into the book I Am That. This book is also available in audio on YouTube and read by Lomakayu.