We may note briefly how the union of heaven with the world occurs by means of correspondences. The Lord’s kingdom is a kingdom of purposes that are functions or—which amounts to the same thing—of functions that are purposes. For this reason, the universe has been so created and formed by the Divine that functions can clothe themselves in materials that enable them to present themselves in act or in results, first in heaven and then in this world, and so step by step all the way to the lowest things in nature. We can see from this that the correspondence of natural phenomena with spiritual ones, or of the world with heaven, takes place through functions, and that the functions are what unite them. We can also see that the forms that clothe the functions are correspondences and unions to the extent that they are forms of the functions.
In the three kingdoms of earthly nature, all the things that happen according to the design are [outward] forms of their functions or results formed by function for function. This is why the things that occur there are correspondences.
As for us, though, our acts are services in forms to the extent that we live according to the divine design—that is, in love for the Lord and in thoughtfulness toward our neighbor. To that extent, our acts are correspondences that unite us to heaven. In general terms, loving the Lord and our neighbor is being of service.
We need to know as well that it is humankind through which the natural world is united to the spiritual world, that we are the means of the union. For there is within us a natural world and also a spiritual world (see Section 57); so to the extent that we are spiritual, we are a means of union. However, to the extent that we are natural and not spiritual, we are not a means of union. The Lord’s inflow into the world and into the world’s gifts within us continues even without our aid, but it does not come into our rational functioning.
from Heaven and Hell, Section 112