Bhagavad Gita Verse 60, Chapter 18

svabhaavajena kaunteya nibaddhaha svena karmanaa |
kartum nechhasi yanmohaatkarishyasyavashopi tat || 60 ||
 
Being bound by your own duty arising out of your nature, O Kaunteya, you will helplessly do that which you do not want to do now, due to delusion.
 
svabhaavajena : arising out of nature
kaunteya : O Kaunteya
nibaddhaha : being bound
svena : own
karmanaa : duty
kartum : do
na : do not
ichhasi : want
yat : which
mohaat : delusion
karishyasi : do
avashaha : helpless
api : certainly
tat : that
 
As Sant Jnyaneshwar says in his commentary, it is impossible for the westward current in a river to flow eastward, and a seed planted in fertile land to not germinate. There are other such examples in the world which illustrate the impossibility of suppressing one's own natural tendencies. Bookies that are sentenced to prison start operating gambling dens inside the prison itself. Unethical businessmen who get elected into political office sell election seats to the highest bidder.
 
Shri Krishna continues to convince Arjuna that the decision to quit the war will not work. Arjuna is the perfect embodiment of a kshatriya, a warrior, possessing all the qualities listed earlier in the chapter. Quitting the war would temporarily have suppressed his fighter instinct, but only temporarily. In due course of time, the force of his vaasanaas, the force of his mental impressions would have impelled him to fight the war he had fled. Worse still, shutting off his fighter instincts through coercion would have driven him to insanity.
 
Arjuna would probably have been convinced about the need to fight the war, since it was clear that he could not walk away from fulfilling his duty, and that he could not forcefully choke his inherent warrior instincts. If we were to take this argument to its conclusion, it means that we are helpless under the influence of our natural tendencies. But there has to be way out of this, otherwise there is no scope for liberation. Shri Krishna answers this doubt through an illustration in the next shloka.