The Buddha explained that it is the five hindrances that distort perception and corrupt our thinking. He called the five hindrances the nutriment that feeds delusion.
The first hindrance, sensual desire, selects what we want to see, hear, sense, and cognize. It often embellishes the truth. It presents to our consciousness the product of wishful thinking.
The second hindrance, ill will, is the negative impulse that blocks us from seeing, hearing, sensing, or cognizing what we don’t want to know. It blinds us to what is unpleasant, and to what is contrary to our view. Psychology knows the second hindrance as the process of denial.
The third hindrance is sloth and torpor. This does not distort what we see, hear, sense, or cognize; rather, it buries it in a fog so that we are unable to discern clearly.
The fourth hindrance, restlessness and remorse, keeps our senses on the run, so fast that we do not have sufficient time to see, hear, sense, or cognize fully. Sights do not have time to fully form on our retina before the back of the eye has another sight to deal with. Sounds are hardly registered when we are asked to listen to something else. The fourth hindrance of restlessness, and its special case of remorse (inner restlessness due to bad conduct), is like the overdemanding boss in your office who never gives you enough time to finish a project properly.
The fifth hindrance is doubt, which interrupts the gathering of data with premature questions. Before we have fully experienced the seen, heard, sensed, or cognized, doubt interferes with the process, like a cocky student interrupting the teacher with a question in the midst of the lecture.
It is these five hindrances that distort perception, corrupt thinking, and maintain a deluded view.
It is well known among serious students of Buddhism that the only way to suppress these five hindrances is through the practice of jhana. ... for those who do not attain a jhana, the five hindrances (plus discontent and weariness) invade the mind and remain. Anything less than jhana is not powerful and lasting enough to suppress the five hindrances sufficiently. So, ... if the five hindrances are still active at a subconscious level, you are not seeing things as they truly are; you are only seeing things as they seem, distorted by these five hindrances.
Thus, ... in order that “in the seen will be merely what is seen, in the heard will be merely what is heard, in the sensed will be merely what is sensed, and in the cognized will merely be what is cognized,” the five hindrances have to be suppressed and that means jhana!
Ajahn Brahmavamsol. "Degrees of Seeing." Lion's Roar magazine.