Brahman is the ultimate, both transcendent and immanent, the absolute infinite existence, the sum total of all that ever is, was, or ever shall be. For Advaita (non-dual) philosophers Brahman is not a God in the monotheistic sense, as they do not ascribe to it any limiting characteristics, not even those of being and non-being, and this is reflected in the fact that in Sanskrit, the word Brahman has two genders (masculine, Brahmâ, the creator-god or Brahman, neuter, the Absolute). Dvaita philosophy holds that Brahman is ultimately a personal God, Vishnu, or Krishna (brahmano hi pratisthaham, I am the Foundation of Brahman Bhagavad Gita).

The Upanishads are speculative Vedic texts that contain elaborations in peotry abd verse. They have a universal feel that has led to the explanation of One Brahman and the inner Atman(Self). They are believed to be composed since 500 BC.The Upanishads are summed up in one phrase- "Tat Tvam Asi", (Thou Art That) by the Advaita Vedanta and they believe that in the end, the ultimate, formless, inconceivable Brahman is the same as our soul, Atman. We only have to realize it through discrimination. The Upanishads also contain the first and most definitive explications of aum or Om as the divine word, the cosmic vibration that underlies all existence and contains multiple trinities of being and principles subsumed into its One Self. "Aum Shanti Shanti Shanti" This, too, is found first in the Upanishads, the call for tranquility, for divine stillness, for Peace everlasting.

Philosophical Development from Upanishadic Metaphysics to Scientific Realism: The Upanishadic period gave way to an era, which was not opposed to the development of rational ideas, even encouraging scientific observation and advanced study in the fields of logic, mathematics and the physical sciences. Following an era when rituals and superstitions had begun to proliferate, in some ways the Upanishadic texts helped to clear the ground for greater rationalism in society. Brahmin orthodoxy and ideas of ritual purity were superseded by a spiritual perspective that eschewed sectarianism and could be practiced universally, unfettered by an individual`s social standing. Much of the emphasis was on discovering "spiritual truths" for oneself as opposed to mechanically accepting the testimony of established religious leaders.

The Upanishadic concept of god was more abstract and philosophical. Different texts postulated the doctrine of a universal soul that embraced all physical beings. All life emanated from this universal soul and death simply caused individual manifestations of the soul to merge or mingle back with the universal soul. The concept of a universal soul was illustrated through analogies from natural phenomenon. As a corollary to this theory emerged the notion that even as individual beings might refer to this universal soul - i.e. god in varied ways - by using different names and different methods of worship - all living beings were nevertheless related to each other and to the universal god, and capable of merging with the universal god. This approach thus laid the foundation for egalitarian and non-discriminatory philosophies such as Buddhism and Jainism (as well as non-sectarian streams of Hinduism) that followed the Upanishadic period. As is evident, such an approach was not incompatible with secular society, and permitted different faiths and sub-faiths to coexist in relative peace and harmony. The common theist solution to this philosophical dilemma was to simply reject logic and demand unquestioning faith on the part of the believer. The Upanishadic philosophers attempted to resolve this contradiction by defining god as an entity that extended infinitely in all dimensions covering both space and time. Another philosophical advance of the Upanishadic period was that religion was transformed from the realm of bookish parroting of scriptures to the realm of advanced intellectual debate and polemics. The Upanishadic philosophers did not lay down their conclusions as rigid doctrines or inviolable laws but as seductive parables - sometimes displaying remarkable worldly insight and analytical skill. By attempting to win over their followers through analogies from nature, and by employing the methods of abstract reasoning and debate, they created an environment where dialectical thinking and intellectual exchanges could later flourish.

The Vaisheshika Schoolwas an early realistic school whose main achievement lay in it`s attempt at classifying nature into like and unlike groups. It also posited that all matter was made up of tiny and indestructible particles - i.e. atoms that aggregated in different ways to form new compounds that formed the variety of matter that existed on the earth. Their philosophy was described through the enumeration of the following concepts: Dravya (Substance), Guna (Quality), Karma (Action), Samanya (Generality), Visesa (Particularity), Samavaya (Inherence) and abhava (non-existence). Dravya (or substance) was understood as the specific result of a particular aggregate effect - i.e. the combination of atoms in a unique way. Substances were repositories for qualities and actions. Guna or quality was that which resided in a dravya. Action or Karma represented physical movement. Unlike quality which was passive, Karma was dynamic. Satta or physical existence was viewed as being the common attribute of substance, quality and action - i.e. only existing (as opposed to imaginary) entities could have substance, qualities and be capable of action.

Samanyata or `generality` was seen as a mental construct to create common classes of substances, qualities or actions while Visesata (particularity) was used to identify and separate individual items from their general classes.

The Vaisheshika school thus served as an important step in the study of science by enumerating concepts that could further the study of physics and chemistry. The Nyaya schools complemented and built on the Vaisheshika school by elaborating on the process of accumulating valid scientific knowledge through accurate perception and generating valid inferences.

The school articulated four means of acquiring valid knowledge: pratyaksha or perception through one of the senses; anumana or inference; upamana or comparison with a well-known object; or shabda - verbal testimony.

A general methodology of ascertaining the truth (tattva) was described which consisted of describing a proposition (uddesa), the ascertainment of essential facts obtained through perception, inference or induction (laksan or uppa-laksana), and finally examination and verification (pariksa and nirnaya). The Nyaya School identified various types of arguments that hindered or obstructed the path of genuine scientific pursuit, suggesting perhaps, that there may have been considerable practical resistance to their unstinting devotion to truth-seeking and scientific accuracy. The Nyaya School also recognized that intelligent and meaningful debates were not possible if certain fundamental principles and basic definitions and concepts were not mutually accepted. The Nyaya School defined a very sophisticated school of rational philosophy where the process of scientific epistemology was analyzed and all the dangers of unscientific reasoning was skillfully exposed.