The obstacles to samadhi
The first step towards defeating an enemy is to know exactly who the enemy is. So, what are the obstacles to the attainment of samadhi?
If one has a strong enough motivation to want to definitely attain samadhi, then one's mind will be directed to its object with real force. The first obstacle is then overcome - the obstacle of motivation. Mental agitation and mental dullness are now the main obstacles to further progress.
Agitation is when the mind wanders to objects of attachment: - worries, fears, and pleasures. With coarse agitation the mind becomes so occupied with some distraction that the original course of thinking is entirely forgotten. The subtle form is more difficult to identify, and occurs when only a small part of the mind is directed elsewhere. Whenever happiness suddenly arises during meditation, it is a result of subtle agitation.