The Function of the mind’s rational level, as Swedenborg defines it, is to be the bridge between what is natural and what is spiritual. For that reason, in order to open the rational level of our mind, we need to have at least some knowledge of the Word of God, which tells us about the spiritual world and things spiritual. Without such instruction, we would never know anything beyond this natural world and its laws. We would be completely in the dark about the supra-natural level of our mind, which is above the world of nature. We would have no concept of the relationship between life in the natural world and life in the spiritual world, because for us in our ignorance the spiritual world and spiritual things would be seem to be nonexistent. In that case, we would not be able to live according to spiritual laws; we cannot live what we do not know.
The rational level of our mind develops in adolescence; it is the point at which we must at least try to start living in accordance with spiritual laws. However, the rational level does not open to the higher levels of our mind, the internal mind, unless we choose to begin the process of regeneration by cooperating with the Lord.
from The Hidden Levels of the Mind, Swedenborg’s Theory of Consciousness by Douglas Taylor