The eighth chapter is called "Akshara Brahma Yoga" or "The Imperishable Brahma in the light of Yoga". Krishna tells Arjuna about the five elements (Adhibhuta) of the physical world (which are the basis of Sankhya philosophy); then He explains what relates to God (Adhidaiva) and how the two concepts are linked together through the idea of Sacrifice (Adhiyajna).
Those who follow the path of the physical world are born again and again for they have chosen to belong in the manifest world and so must become manifest repeatedly; but those who follow the path of God will cease to be reborn when they achieve perfection in their chosen path.
What is Brahma, and Adhyatma,
And tell me thou what is Adhiyajna;
The Blessed Lord said:
Brahma is the unchangeable;
His perishable form's Adhibhuta;
At time of death, remembering me,
Whatever form remembers he,
Therefore do thou at all times
By practice perfect made in Yoga,
Omniscient, ancient, ruler of the world,
Who meditates on Him at time of death,
I will tell the briefly all about that place,
Controlling all the senses, who
Remembering me, and uttering Om,
I am easily attained by him,
Attaining me, the high-souled ones,
Up to the world of Brahma, all
They who know a day of Brahma
At day's approach, the manifest
This multitude of beings all,
Beyond this life unmanifest,
The changeless and the unmanifest
The Lord supreme maybe attained
And now, O best of Bharata race,
Fire, light, and day, the bright fortnight,
And smoke and night, the dark fortnight,
These are the paths, the bright and dark,
Knowing these paths, O Pritha's son,
What holy fruit can be obtained by alms,
Brahma and Adhyatma: The innate nature of Brahma refers to the soul; that is to say, Brahma, the Lord, is the soul.
Sacrifice and action: Sacrifice is here defined as action which creates and supports life.
Brahma and Adhibhuta: The perishable form of God refers to the elements; that is to say, the five elements and all that belongs to them are perishable, though created by God. They perish in the sense that they change from one form to another, and so the previous form may be said to perish.
Adhidaiva is Purusha: The supreme Purusha is said to be the Lord, or what relates to him.
Adhiyajna and Krishna: The idea of the supreme Purusha of Vedanta Is born- out of Sacrifice, as we have explained; and so Krishna may be identified with Sacrifice; and it is in this form that he dwells in the body of each created being. In other words, whenever an act of Sacrifice is performed by a person, it implies that Krishna is in him.
Remembering Krishna: Whenever we think of God, he is with us, and so we go unto him.
Controlling the Mind and Breath: These are Yoga practices by means of which one is said to attain to whatever he desires.
Om and Brahma: The syllable Om is closely associated with Prana or Breath, the energy of the Soul or Brahma. Hence Brahma is here identified with Om, as in an earlier place he is with the Soul. We have pointed out that Brahma is the supreme Purusha of all systems of thought, including Vedanta, in the Upanishads; and Vedanta is based on the character of the Soul.
The Worlds of Brahma: Here Brahma is conceived in a different sense to that of the God of Vedanta; and so his world is said to be inferior to that of Krishna, the deity of that system. Brahma is the deity of Nyaya and Vaiseshika systems of thought as well.
The day and night of Brahma: Innumerable years are said to make the day and night of Brahma. It is said that a year of men is equal to a day and night of the Gods. Twelve thousand such years form a cycle of time, consisting of the four Yugas or Ages, Satya, Treta, Dvapara, and Kali; and a thousand such cycles form a single day of Brahma. The idea is that Time is almost infinite.
Creation and dissolution of the universe: Day is symbolic of Purusha or God, and night of Prakriti. All life belongs to the former, and death to the latter. Hence we are told that all life is created when the day of Brahma dawns, and it comes to an end when his night approaches.
Beyond the world of Brahma: The highest system of thought is Vedanta, so the highest God is Krishna, its deity. Hence the world of Krishna is higher than that of Brahma.
The two paths - the dark and bright: All life is rendered in terms of Purusha and Prakriti. The one is symbolized in terms of light, and the other of darkness; and life belongs to Purusha, and death to Prakriti.
Hence those who pass away when there is light, go to Purusha; but those who die when darkness prevails, go to Prakriti, or the world of the manifest. And so the latter are born again and again. Those who are merged in God, are perfect, and so have no need to be reborn.