Anyone who does not know how the divine design is arranged in levels cannot grasp how the heavens are distinguished from each other, or for that matter, what the inner person and the outer person are (in an individual). The only idea most people in this world have about inner and outer things is one of continuity, or of a coherence along a continuum from the finer to the coarser. Inner and outer things are not arranged in a continuum, though, but with definite boundaries.
There are two kinds of levels, continuous and noncontinuous. Continuous levels are like decreasing levels of light from a flame, all the way to darkness, or like decreasing amounts of sight from objects in the light to objects in the shade, or like levels of density of the atmosphere from the lowest to the highest. These levels are measured by distance.
Noncontinuous or distinct levels, though, are separated like prior and posterior, cause and effect, producer and product. Anyone who looks closely will discover that there are these kinds of stages of production and composition in everything in the world, no matter what, with one thing arising from another, and a third from that, and so forth.
People who do not acquire a grasp of these levels have no way of knowing how the heavens are arranged or the arrangement of our own deeper and more outward abilities, or the difference between the spiritual world and the natural world, or the difference between our spirit and our body. This also means they cannot understand what correspondences and images are or what inflow is like. People who are attentive only to their physical senses do not grasp these differences, but regard them as instances of increase and decrease on the model of continuous levels. As a result, they cannot think of the spiritual except as a kind of purer natural; so they stand outside, far removed from intelligence.
from Heaven and Hell, Section 38