oordhvam gacchanti sattvasthaa madhye tishtanti raajasaahaa |
jaghanyagunavrittisthaa adho gacchanti taamasaahaa || 18 ||

Those established in sattva go upwards, those in rajas stay in the middle. Those under the influence of the lowest guna, established in tamas, go downwards.
oordhvam : upwards
gacchanti : go
sattvasthaa : established in sattva
madhye : middle
tishtanti : stay
raajasaahaa : established in rajas
jaghanya : lowest
gunavrittisthaahaa : influence of guna
adhaha : downwards
gacchanti : go
taamasaahaa : established in tamas
Parents are always watchful of their children’s behaviour, because the values that are inculcated in childhood stay with us throughout our life. If parents notice that their child is lazy and remains idle all the time, they will first motivate him through selfish desires. They will teach him that if he studies hard and works hard, he will be able to buy fast cars, electronic gadgets, a big house and so on. Once he has risen from a tamasic state to a rajasic state, and has made enough money, his parents will encourage him to slowly start transitioning from rajasic action to selfless, sattvic actions like donating money to charity, volunteering and so on.
Shri Krishna gives us a similar roadmap for our evolution in this shloka. He says that once we have determined our degree of rajas and tamas, we should consciously perform the actions necessary to uplift our mental state. If we are primarily tamasic in nature, we should perform rajasic actions. If we are primarily sattvic in nature, we should perform sattvic actions. He also puts the responsibility of self improvement squarely on our shoulders. No other person can make this happen. The will to self improve must come from within.
Let us imagine for a moment that we are able to uplift ourselves to the level of sattva, where we are able to remain in a sattvic state for a majority of the day, as a consequence of performing sattvic, selfless actions. Is this our goal, or is this yet another milestone in our spiritual journey? The thirteenth chapter taught us that we have become the individual soul, the jeeva, the Purusha, by forgetting our true nature as the eternal essence. We have further become entangled in Prakriti by identifying with a mind and body that are under the influence of the gunas. Once we are able to stay in sattva, and minimize the effects of rajas and tamas, how do we then disentangle ourselves from Prakriti? Shri Krishna picks up this topic next.