Sankhya Yoga 2.28

Verse 2.28

अव्यक्तादीनि भूतानि व्यक्तमध्यानि भारत |

अव्यक्तनिधनान्येव तत्र का परिदेवना || 28||

avyaktādīni bhūtāni vyakta-madhyāni bhārata

avyakta-nidhanānyeva tatra kā paridevanā

avyakta-ādīni—unmanifest before birth; bhūtāni—created beings; vyakta—manifest; madhyāni—in the middle; bhārata—Arjun, scion of Bharat; avyakta—unmanifest; nidhanāni—on death; eva—indeed; tatra—therefore; kā—why; paridevanā—grieve

O scion of Bharat, all created beings are unmanifest before birth, manifest in life, and again unmanifest on death. So why grieve?

The definition of birth and death is given in this verse.

Everything that is created must have a cause for its creation. The cause of the creation of a pot is clay. The causal state is the state before the appearance of a thing. This state is called avyaktam, the unmanifest form. The cause undergoes a change to produce objects out of it. This state is called the manifest state (vyaktam). If seed is the cause, the appearance of roots, branches and leaves is called manifest state. This manifest state can be experienced by the sense organs. 

The five elements are the cause for the manifestation of the physical body. After death the physical body goes back into the five elements. The appearance, sustenance and the dissolution happen in the five elements. The subtle body goes back into the causal body.

In the same manner, the dissolution of the universe into the lord is called avyaktam. In reality, nothing undergoes destruction in this world. Everything oscillates between the manifest and the unmanifest states. When something comes into manifest state we say it is born and when it goes back to the unmanifest form we say it is dead. No one can create anything that is not already present. What came into existence was already present in a different form. Then it goes back to where it came from. Hence There is no birth nor is there death. Knowing this one must not grieve.