The Buddha taught people about each of the six doors so that realities could be known separately, one at a time. We usually confuse all the doorways and there is no mindfulness of only one reality at a time when it appears.
We read in the “Kindred Sayings” (IV, Sarayatana-vagga, Kindred Sayings on Sense, First Fifty, Ch I, §1) that the Buddha, while he was staying near Savatthi, at the Jeta Grove, taught the impermanence of the eye. He said to the monks:
“ ... The eye, monks, is impermanent. What is impermanent is dukkha. What is dukkha, that is anatta. What is anatta that is not mine, I am not it, it is not myself. That is how it should be seen with perfect insight as it really is.”
The same is said about the ear, the nose, the tongue, the body and the mind. The sutta then continues:
“ ... So seeing, monks, the well-taught ariyan disciple is repelled by the eye, the ear, the nose, the tongue, the body and the mind. Being repelled by them, he is not attached to them. Being not attached to them he is freed. In this freedom comes the insight of being free. Thus he realizes: ’Rebirth is destroyed, lived is the righteous life, done is the task, for life in these conditions there is no hereafter.’ ”
Each reality has to be known separately. The Buddha taught the true nature of each reality which appears.