Understanding and thinking are totally different. Wisdom understands and knows clearly what is happening and how it happens. Assumption does not understand, it only thinks what it wants – it is delusion. Thinking and understanding are opposites.
To know clearly means to understand. On the other hand, you think because you don’t know.
Sometimes yogis don’t know why meditation is good or bad. They then assume that it is because of the place, time, surroundings, or something else. They think what they want, and then they believe what they have thought out.
This is the wrong process. Whether meditation is good or bad depends on your attitude - right thought or wrong thought. Wisdom is right thought and right view; defilement is wrong thought.
Meditation is not good due to defilement, and meditation gets better and better because of right thought and right view.