There is an outside and an inside to each heaven. The angels who are in the inner region are there called “inner angels,” while the ones in the outer region are called “outer angels.” The outside and the inside in the heavens (or in each particular heaven) are like our own volitional side and its cognitive aspect. Everything volitional has its cognitive side—neither occurs without the other. The volitional is like a flame and the cognitive like the light that it sheds.
It needs to be quite clear that it is the inner nature of angels that determines which heaven they are in. The more the deeper levels have been opened, the more inward the heaven they are in. There are three inner levels of every angel and spirit, and of every person here as well. The people whose third level has been opened are in the central heaven, while the people whose second or first only has been opened are in the intermediate or the outmost heaven.
The deeper levels are opened by our acceptance of divine good and divine true gifts. People who are actually affected by divine true gifts and let them directly into their lives—into their intentions and therefore into act—are in the central or third heaven, located there according to their acceptance of what is good in response to truth. People who do not let such gifts directly into their intentions, but into their memory and from there into their discernment, intending and doing them as a result of this process, are in the intermediate or second heaven. People who live good moral lives, though, and believe in the Divine with no particular interest in learning, are in the outmost or first heaven. We may therefore conclude that the state of our inner natures is what constitutes heaven and that heaven is within each one of us, not outside us. This is what the Lord teaches in saying,
The kingdom of God does not arrive when we are looking for it, nor do they say, “Here it is,” or “There it is.” Behold, you have the kingdom of God within you. (Luke 17:20–21)
from Heaven and Hell, Section 32, 33