Now, there is a simple description, which seems very simple, but you see the sweep of it:
bhūmir āpo ’nalo vāyuḥ khaṁ mano buddhir eva ca |
ahaṁkāra itīyaṁ me bhinnā prakṛtir aṣṭadhā || (VII, 4)
This is a description of the whole world: what we call physics, chemistry, biology, if you want to analyse all that in essence, basically, if you want to understand even psychology basically, knowledge of matter, life and mind, as we understand in the world, is described in two sentences:
bhūmi, earth; āpo, water; anala, fire; vāyuḥ, wind; khaṁ is the sky, is the ether; then manaḥ, mind; buddhi, intellect; ahaṁkāra egoism. All these 8 elements put together is the bhinnā prakṛtir aṣṭadhā, it is the Prakriti in which there is all kinds of divisions, every word has a meaning: prakṛtir aṣṭadhā, so there is first a word called Prakriti, this is the first starting point of jñānaṁ savijñānam: the knowledge of the essence and the knowledge of all the details.
And you can start with aṣṭadhā prakṛtir. Prakriti itself is the word which means: ‘that which put forth energy of action’. So it says first of all, that “there is in this world energy put forth for action: that is Prakriti; it is bhinnā, it is the energy in which there are lots of divisions. In the action of energy, there are lots of divisions, ‘dividing Prakriti’ you might say, Prakriti itself is something, which constantly divides.
In other words, if you look for unity, you will not find in Prakriti; we used the word Prakriti only one as if it is unifying, but real unifying force, unifying reality is not in Prakriti. This Prakriti is eight fold. In this eight-fold, five principles are physically visible in some degree or the other. But three are psychological principles. The five are very well known in India, they have all been called: pañca mahābhūtas, they are the five big physical principles, starting from ākāśa, that is called “kha” in Sanskrit (kha is also Akasha); then comes vāyu, the air; then comes agni, the fire; then comes jala, the liquid principle; and then comes pṛthivi, the physical, purely physical that which you can touch. The others are also physical, but earth is something that is characterised by something that reacts, response to you in a solid manner, which response with a striking force, regarding which you can say that action and reaction are equal and opposite, that is Prithvi, the physical principle.
So, it tells us that the whole world you can examine, its multiplications are hundreds and millions, but basically, nothing in the world of physical reality can go beyond this five principles: this is tattvataḥ, essentially told about the whole world in one single sentence: that the whole physical world is nothing but these five principles. And all that is psychological in our ordinary consciousness is in 3 words: manaḥ, buddhi, ahaṁkāraḥ. In your whole psychology there is nothing but manaḥ, the mind; buddhi, intellect; and ahaṁkāraḥ the sense of ego.
Now, you will see that Sri Krishna does not explain what is manaḥ, what is buddhi, what is ahaṁkāraḥ, what are the distinctions; that is because it is already known. In any case He leaves it to the seeker to find out the exact meanings of all this.