Since a human being is a heaven and a world in least form in the image of the greatest (see Section 57), there is a spiritual world and a natural world within each of us. The deeper elements, which belong to our minds and relate to our intelligence and volition, constitute our spiritual world, while the outer elements, which belong to our bodies and relate to our senses and actions, constitute our natural world. Anything that occurs in our natural world (that is, in our bodies and their senses and actions) because of our spiritual world (that is, because of our minds and their intelligence and volition) is called something that corresponds.
We can see in the human face what correspondence is like. In a face that has not been taught to dissimulate, all the affections of the mind manifest themselves visibly in a natural form, as though in their very imprint, which is why we refer to the face as “the index of the mind.” This is our spiritual world within our natural world. Similarly, elements of our understanding are manifest in our speech, and matters of our volition in our physical behavior. So things that occur in the body, whether in our faces or in our speech or in our behavior, are called correspondences.
We can also see from this what the inner person is and what the outer person is, namely, that the inner is the one that is called the spiritual person, and the outer the natural person. We can also see that they are as distinct from each other as heaven and earth, and that everything that happens and comes forth in the outer or natural person does so from the inner or spiritual one.
from Heaven and Hell, Sections 90-92