Some of the earliest philosophical writings in existence have come to us through the Indian philosophers in the Vedas and the Upanishads which far outdate all other known sources. Professor Chandradhar Sharma writes in his book A Critical Survey of Indian Philosophy, Chapter 1, Section Introduction:
The etymological meaning of the word 'philosophy' is 'love of learning'. It signifies a natural and a necessary urge in human beings to know themselves and the world in which they 'live and move and have their being'. It is impossible for man to live without a philosophy. The choice, as Aldous Huxley puts it, is not 'between metaphysic and no metaphysic; it is always between a good metaphysic and a bad metaphysic'.
Western Philosophy has remained more or less true to the etymological meaning of 'philosophy', in being essentially an intellectual quest for truth. Indian Philosophy has been, however, intensely spiritual and has always emphasized the need for practical realization of truth. The word 'darshana' means 'vision' and also the 'instrument of truth'. It stands for the direct, immediate and intuitive vision of Reality, the actual perception of Truth, and also includes the means which lead to this realization. 'See the Self' [See meaning to know and not in an intellectual manner, Self meaning the One Existence] (atma va drastavyah) is the keynote of all schools of Indian Philosophy. And this is the reason why most of the schools of Indian Philosophy area also religious sects...
The Vedas are the oldest extant literary monument of the Aryan mind. The origin of Indian Philosophy may be easily traced in the Vedas. Indian Philosophy, as an autonomous system, has developed practically unaffected by external influences...
The name 'Veda' (knowledge) stands for the Mantras and the Brahmanas...The appendages to these Brahmanas are called Aranyakas mainly because they were composed in the calmness of the forests. The Aranyakas mark the transition from the ritualistic to the philosophic thought. The concluding portions of the Aranyakas are called the Upanishads. They are intensely philosophical and spiritual and may be rightly regarded as the cream of the Vedic philosophy. The Mantras and the Brahmanas are called the Karma-Kanda or the portion dealing with sacrifical actions, and the Aranyakas and the Upanishads are called the Jnana-Kanda or the portion dealing with knowledge...
In his translation of the Brahma-Sutras, Swami Vireswarananda writes (pp ii-iii) (online available here - https://www.wisdomlib.org/hinduism/book/brahma-sutras/d/doc62753.html):
Though we find Vedântic thought even in some of the earliest hymns of the Rig-Veda, e.g. the Nâsadiya Sukta, which forms as it were the basis of later Upanishads, yet there is no denying the fact that the Indo-Aryans in their earlier days in India were given more to rituals and sacrifices. These were elaborated to such an extent by the Brâhmanas, the priestly class, that persons of rationalistic bent of mind revolted and questioned the very efficacy of the sacrificial religion. They engaged themselves in metaphysical problems and arrived at different solutions of the world. The Vedântic thought that was in germ form was now developed more and more, and we have the Upanishads. This spirit of revolt against ritualism was carried on mainly by the Kshatriyas. The Indo-Aryans were very bold thinkers and nothing was sacrilegious to them in their search after truth. Traces of opposition against the religion of the Vedas are found in the Vedas themselves. This tidal wave of rationalism in its extreme form gave rise to such schools of thought as the Chârvâkas [comparable to modern day atheism], which were extremely materialistic and anti-religious.
In the age immediately preceding Buddha and during his lifetime there was a great religious and philosophical upheaval in India. From the Brahma-jâla-Sutras we learn that in his time there wrere as many as sixty-two different schools of philosophy in India. We also learn from Buddhistic literature the names of a good number of teachers who were venerated in Âryâvarta at the time—names like Purâna Kasyapa, Kâtyâyana, Makkâli Gosâla, Nigantha Nâthaputra, the founder of Jainism, and others. While these great souls represented Indian culture from an anti-Vedic standpoint there were many great names that represented the culture from the traditional standpoint—names that are still venerated by Hindu religion and culture.
The destructive criticism of everything in the old system by the Chârvâkas and others set the orthodox section to organize their belief on a more rationalistic basis and render it immune against all such criticism. This led to the foundation of the six systems of orthodox Hindu philosophy—orthodox in the sense that they accepted the authority of the Vedas in things transcendental—while there were others who did not accept this authority and therefore were dubbed heterodox, though otherwise they too were the outcome of Upanishadic thought. The acceptance of the authority of the Vedas by these orthodox schools, however, does not mean that they accepted them in toto . Their allegiance to the Vedas varied widely and often it was too loose. Of the six orthodox schools, viz. Nyâya, Vaiseshika, Sânkhya, Yoga, Purva Mimâmsâ and Uttara Mimâmsâ or Vedanta, the last two are intimately connected with the Vedas, which is one of the reasons why they are not mentioned in the Jaina and Buddhistic literature, while the others are mentioned.
These six orthodox systems of thought developed side by side at different intellectual centres, of which there were a good number all over the country even during the Upanishadic period. Again in each system there were shades of difference. Thus for centuries philosophic thought developed in India till at last it became so unwieldy that a regular systematization of each school of thought was found a great necessity. This led to the Sutra literature.
A study of the Upanishads shows that Eastern philosophical reasoning was in full bloom well before the Greeks.