Reality is illusion while illusion is reality. Both of these opposites are part of the whole. Only the state of our mind determines what kind of world we live in. With this knowledge we have the power and freedom to create our own worlds.
The circular inscription on the collection's pieces is a reminder that whatever situation we find ourselves in, we can always be free of it. After all, it is only a temporary reality, the importance of which depends on our attitude toward it. It is up to us to decide whether reality is illusory or illusion is real. When we remember this, then we are able to pass through any difficulties along the way.