The Three Circles of Existence can be traced back to Edward Williams (Iolo Morganwg by his bardic name) who claimed they were part of ancient druid tradition but are largely accepted to be inventions of his own. Still, much of his work has contributed to modern druidic tradition, such as the Druid’s Prayer, and even the Three Circles can provide inspiration for modern druidry.
The Three Circles through which a soul journeys are Annwn, Abred and Gwynvid. Finally there is Ceugant which represents the Infinite, God or Spirit. The journey of the soul begins in Annwn, the Cauldron of Rebirth. From here souls are incarnated in the physical world, Abred, and when the body dies the soul returns to Annwn with the lessons learnt. The soul continues evolving through mineral, vegetable, animal and human forms, until finally it reaches a spiritual perfection where it may then enter heavenly realm of Gwynvid, from where it need not return.
In naturalist terms this doesn’t hold water, but I have found a parallel in Teilhard de Chardin’s work on cosmology: Annwn represents the geosphere (inanimate matter), Abred represents the biosphere (biological life), and Gwynvid represents the noosphere (human consciousness and its development). Annwn is the entire material universe, within which we can find the living world of Abred, and from that evolves Gwynvid.
There is a constant “reincarnation” or recycling of matter through biological lifeforms: soil is consumed by a plant, incorporated into the plant as a leaf, the leaf then dies and returns to soil (or it is eaten and recycled/reincarnated through the food chain). Life evolves, and from it appears the noosphere, specifically human consciousness.
For me, this fits with the Gaian worldview (see Gaia Theory by James Lovelock), where the planet as a living system has developed a mind through humanity, which forms a network of consciousness (noosphere). At present the noosphere is disparate and even volatile, and certainly not a harmonious contributor to the ecosphere, but we can see through history how it and human culture have developed, and how ideas have travelled and helped form a globalisation of culture (and with the advent of the Internet, information can now travel round the world at the blink of an eye, making the Internet function like a nervous system for the noosphere). As each individual develops noetically, the noosphere in turn evolves, becoming more refined and coordinated, and it becomes another part of the evolution of Gaia, a composition of Annwn, Abred and Gwynvid.
Ceugant is a bit more difficult to include, but we could say it parallels Teilhard’s Omega Point, the apex of the evolution of consciousness. Ceugant stands beyond what we can know, and perhaps represents an “infinite mystery”. We may never reach beyond the known, but there is always the possibility that the unknown can, progressively, become known. There are plenty of horizons of potential!