In the permanent stone circles of the Megalithic era, where rituals were enacted for hundreds of years, great reservoirs of power were built up. Because the stones defined the space, there was no need to "draw the circle" as witches and shaman do. The form of circle casting that we use today was probably originated during times of persecution, when meetings were held secretly, indoors, and it became necessary to create a temple in a simple hut.
When we cast a circle we create an energy form, a boundary that limits and contains the movements of subtle forces. The outer forms are a cloak for inner visualizations, so the circle becomes a living mandala, in which we are centered.
In Witchcraft and Shamanism, the function of the circle is not so much to keep "out" negative energies as to keep "in" power so that it can rise to a peak. You cannot boil water without first putting it in a pot, and you cannot raise power effectively unless it is also contained.
Leaving the circle during a ritual is discouraged, although cats and small children seem to pass across without disturbing the force field.
The casting of the circle is the formal beginning of the ritual, the complex "cue" that tells us to switch our awareness into a deeper mode. In ritual we "suspend disbelief" just as we do when we are watching a play.
We allow the critical and analytical functions of our waking or "Talking Self" to relax so that our primal or "Younger Self" may respond fully and emotionally to what happens. The "Younger Self" responds best to actions, symbols, tangibles - so this change in consciousness is acted out, using a rich array of tools and symbols.
Casting a circle is an enacted meditation. Each gesture made, each tool used, each power invoked, resonates through layers of meaning to awaken an aspect of ourselves.
Brightest of Blessings,