This article is a close translation, with explanatory notes, of the treatise Tattuva-Kattalei, the law of things according to their essential nature. The notes which are dispersed throughout the treatise will render the whole more intelligible and readable, and help to bring the system more distinctly to view. The meaning of the term ‘tattuvar’ (singular of tattuvam) is essential nature, or property, of anything. It is used to designate any essential part of the human organism. It has no equivalent in English. The ideas wrapped up in the tattuvam confound the physical and the metaphysical. The Tattuva-Kattalei is more systematic and complete than any other of the smaller treatises. It presents the standard system of the orthodox Saivas of southern India and Ceylon. This treatise was probably designed as a guide or manual for the Guru. It is too brief to give any intelligible view of the system to the uninitiated.