From an Ayurvedic perspective, human constitution is composed of three forces in unique combinations; namely Vata(which is represented by Air), Pitta (represented by Fire) and Kapha (represented by water). Due to this, no two individuals are alike. Based on individual constitution, various factors like foods, spices, emotions, climates, colors, herbal medicines and activies, etc., leads to a balanced or imbalanced state of an individual and these factors also play an important role in improving or worsening the illness in an individual.
The books on four Vedas i.e., Rigveda, Sama Veda, Yajur Veda and Atharva Veda includes topics based on health, spiritual business, army, poetry, astrology, behaviour and spiritual living. Among them Rig Veda being the oldest book has a reference to a cosmology called Sankhya, which provides the base for both Ayurveda and Yoga and explains concepts of health, diseases, pathogenesis and treatment principles. Rig Veda also has discussions based on Vata, pitta and kapha and the importance of herbs in healing diseases pertaining to body and mind and to promote longevity. The Vedic sages compiled books on Ayurveda based on the Vedic scriptures and one such book called Atreya Samhita is the oldest medical book in the world. The Vedic Brahmanas later became the vaidyas (physicians of Ayurveda). Atharva Veda included eight divisions of Ayurveda namely surgery (head and neck), internal medicine, opthamology, Otorinolaryngology, psychiatry, pediatrics, toxicology, Gerontology, science of fertility and science of rejuvenation. During the 16th century Paracelsus, well known as the father of modern western medicine, propagated a system of medicine which was heavily based on Ayurveda. Later people from different countries came to Ayurvedic schools in India, to learn about Ayurvedic medicines and treatment.
Charak and Sushrut were re-organizers of Ayurveda, who developed the third major thesis called Ashtanga Hridayawhich contains the brief version of their works. This led to the formation of three main texts of Ayurveda that are being used till date, namely Charak Samhita, Ashtanga Hridaya Samhita and Sushrut Samhita. These texts are considered as the only complete existing medical system.