Bhagavad-gita 2.16 Seers of truth know the difference between matter and spirit

Seers of truth have concluded that there is no endurance of the non-existent material body and there is no change of the eternal soul.

As per modern medical science, body is changing every moment by the actions and reactions of different cells and thus growth and old age are taking place in the body. Spirit soul exists permanently, remaining the same despite all changes of the body and mind.

jyotimsi visnur bhuvanaani visnuh … Visnu Purana (2.12.38)
Visnu and His abodes have self-illuminated spiritual existence. Hence spiritual world is existent unlike non-existent material world.

This is the beginning of the instruction of the Lord to the living entities who are bewildered by the influence of ignorance. Removal of ignorance involves thorough study of oneself, re-establishing the eternal relationship between oneself as worshipper and Supreme Personality of Godhead as worshipable and understanding the difference between part-and-parcel living entities and the Supreme whole. 

In the Vedanta-sutra as well as the Srimad Bhagavatam, the Supreme has been accepted as the origin of all emanations which are experienced by superior living entities and inferior natural sequences. The Supreme is accepted as energetic and material nature or energy is accepted as subordinate to the Supreme Lord. Hence He is the source of all knowledge. Lord Krishna therefore teaches the Bhagavad-gita for the enlightenment of all living entities for all times.