Humanity goes through successive periods of analysis and synthesis, fragmentation and unification. During periods of fragmentation, the focus is on concentrating in on particular directions, to the exclusion of others, and we find in these cases oftentimes a large number of competing or apparently conflicting directions being advanced by various people. During periods of synthesis, we find the tendency of taking the numerous divergent strands and bringing them together in an emphasis on their underlying unity.
Sri Aurobindo outlines 4 periods of synthesis that mark the history of the spiritual development. These are the Vedic times, the Upanishads, The Bhagavad Gita, and the Tantric synthesis. Each of these represents an especially fruitful period in the progress of humanity toward integrating spirituality into life. The emphasis of each one was based on the needs of the time in which it occurred.
“We start with the Vedic synthesis of the psychological being of man in its highest flights and widest rangings of divine knowledge, power, joy, life and glory with the cosmic existence of the gods, pursued behind the symbols of the material universe into those superior planes which are hidden from the physical sense and the material mentality.”
“The Upanishads…draw together into a great harmony all that had been seen and experienced by the inspired and liberated knowers of the Eternal throughout a great and fruitful period of spiritual seeking.”
“The Gita starts from this Vedantic synthesis and upon the basis of its essential ideas builds another harmony of the three great means and powers, Love, Knowledge and Works, through which the soul of man can directly approach and cast itself into the Eternal.”
“The Tantric…brings forward into the foreground along with divine knowledge, divine works and an enriched devotion of divine Love, the secrets also of the Hatha and Raja Yogas, the use of the body and of mental askesis for the opening up of the divine life on all its planes…. Moreover, it grasps at that idea of the divine perfectibility of man, possessed by the Vedic Rishis but thrown into the background by the intermediate ages, which is destined to fill so large a place in any future synthesis of human thought, experience and aspiration.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 1, Our Demand and Need from the Gita, pp. 7-8,
Sri Aurobindo Studies 