Bhagavad Gita Verse 37, Chapter 1

tasmaannarhaa vayam hantum dhaartaraashtraan svabaandhavaan |
svajanam hi katham hatvaa sukhinaha syaama maadhava || 37 ||
 
Therefore, it is not appropriate for us to kill these relatives of Dhritraashtra who our also our brothers. How can we gain pleasure by killing our own, O Maadhava?
 
tasmaat : therefore
na arhaaha : not appropriate
vayam : we
hantum : to kill
dhaartarashtaan : relatives of Dhritraashtra
svabaandhavaan : our brothers
svajanam : our kinsmen
hi : because
katham : how
hatvaa : kill
sukhinaha : happiness
maadhava : O Maadhava
 
The word "tasmaat", which means therefore, indicates the conclusion of an argument. Here, Arjuna concluded his argument to Shri Krishna in which he asserted that that he did not want to fight. To defend that argument, Arjuna provided several points: that there was no point in the war or even in living itself, that the very people that he was fighting against were the same people that made him happy, that killing his kinsmen and his well-wishers was a sin, and that there would be no joy derived in doing so.
 
This argument was not built on any sort of rationality or logic because Arjuna came under the influence of "moha" or delusion, the delusion that personal relationships were more important in the battlefield than one's duty. An increase in moha usually suppresses our ability to discern between what is correct and what is not. This discerning ability is called "viveka".
 
Here's an real world example that illustrates moha and viveka. Imagine that your brother has a drinking problem, and needs to hear from you that the addiction needs to stop. What is the right thing for you to do? It is a difficult situation because your moha and viveka come into conflict. Viveka tells you that the right thing for you to do is to intervene, but moha tells you that doing so will endanger your relationship.
 
Another common example is that a surgeon will usually not perform an operation on a relative exactly because of this moha.
 
Viveka is the first step in the "Saadhana Chatushthaya", the 4-fold qualifications that are required for anyone treading on the spiritual path. Barring a few exceptions if you do not cultivate the ability to discern what is correct and what is not, your spiritual journey will never commence.