Bhagavad Gita Verse 4, Chapter 13

rishibhirbahudhaa geetam chandobhirvividhaihi prithak |
brahmasootrapadaishchaiva hetumadbhirvinishchitaihi || 4 ||
 
Sages have sung (about the field and its knower) in many ways, it has been stated in various Vedic chants and also in the logical and conclusive texts of the Brahma Sootras.
 
rishibhihi : by sages
bahudhaa : many ways
geetam : sung
chandobhihi : Vedic chants
vividhaihi : various
prithak : distinctly
brahmasootrapadaihi : Brahma Sootra texts
cha : and
eva : also
hetumadbhihi : logical
vinishchitaihi : conclusive
 
Shri Krishna glorifies the knowledge of the field and its knower in this shloka. He affirms that this very same knowledge was first revealed to the rishis or sages, and is not something that has been invented in the Gita. It was then documented in the four Vedas: the Rik, Saama, Yajur and Atharva. It is to be found in all three sections of each of these Vedas: the Samhitaa (hymns), Braahmanaa (theology) and Upanishads (discourses on divine knowledge). It is important to note that this knowledge was revealed to the sages in their meditations, and not authored as an original composition by anybody. Since this knowledge is beyond the realm of the senses and the mind, it could not have come from any one person's mind. It had to come from direct experience arrived at through meditation.
 
Now, even if we are able to access this divine knowledge by studying the Vedas, we cannot assimilate this knowledge without the help of logic. We need something that can explain, step-by-step, what is revealed in the four Vedas. We also need logic to reconcile apparent contradictions that we as students come across while studying the Vedas. The Brahma Sootras, authored by Vyaasa, use logic and reason to present the knowledge of the Vedas in a precise, formula-like method. Each sootra in that text is hardly two or three words long, but is packed with so much information that commentators like Shankaraachaarya write pages and pages to explain just one sootra.
 
The Bhagavad Gita, the Upanishads and the Brahma Sootras are known as the "Prasthaana Traya", and together provide a comprehensive compendium of knowledge about the field and its knower, as well as practical training on how to go about it. Of the three, the Bhagavad Gita is closest to the understanding of a common man. Shri Krishna instructs us to learn about the field and the knower in brief such that we can progress in our spiritual journey. But if we want to delve deeper into the details for academic reasons, we are more than welcome to read the Vedas and the Brahma Sootras. He now proceeds to talk about the field, the kshetra, in detail.