From the introduction:
On March 6th, in the year 2217, Martin Thompson applied a revolutionary combination of mind theory and chemical science, to discover that ours is not the only world. He learned that certain minds, under certain conditions, can be made to shift their perception into other realities. Other worlds. Some of these worlds are much like ours, some are very different. He called these alternate realities phases of existence, and proposed that when a mind transitioned between them, the phases met and touched at that single point, much like soap bubbles floating in air.