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.

Bhagavad-gita 2.14 – 2.15 Seers discharge proper duty indifferent to happiness and distress

As per Vedic civilization, discharge of proper duty must follow prescribed rules and regulations of religious principles. Seers of truth who are best among men consistently discharge their duties properly. They rise to the platform of knowledge and become eligible to liberate themselves from the clutches of maya (illusion) and material bondage.

Such elevated persons learn to tolerate non-permanent appearances of happiness and distress during proper discharge of duty. They are not disturbed by the onslaughts of happiness and distress and remain steady in their determination to attain advanced stage of spiritual realization.

The nonpermanent appearance and disappearance of happiness and distress are like the appearance and disappearance of winter and summer seasons. They arise from sense perception and one must learn to tolerate them without being disturbed.

According to Vedic injunction, one has to take bath early in the morning even during cold winter. A woman does not hesitate to cook in the kitchen even during hot summer. Therefore, one has to execute his duty in spite of climatic inconveniences.

As per varnasrama principle, one has to fight as a ksatriya even if the enemy is some friend or relative. Arjuna was therefore advised by Lord Sri Krishna to discharge his duties without disturbance. Arjuna’s heritage from both his parent’s sides made him responsible to not avoid fighting.

One who is serious about making his life perfect adopts the sannyasa order in spite of difficulties that arise from severing family relationships and from giving up connections with wife and children. Lord Caitanya took sannyasa at the age of 24 leaving his dependent wife and old mother showing the example of how to be steady in the discharge of higher duties.