Most popular for many humans, to this day, is duality. Two worlds. This One and That One.
This One is the world humans experience. It is silly, really, for each human experiences a world unique to that human. Somehow humans tend to believe that they all live in the same world. It is the strangest thing to the point where most other species don't quite understand and cannot quite follow the concept. It is taken for granted that there are as many worlds, at least, as there are perceptions of the world.
That One is slightly more complex, but only slightly. It is thought of as the world beyond perception and it overlaps in points within the human body. Most think it overlaps with the gut, which sometimes seems to know things that the conscious parts of perception of the brain do not. It is strange indeed.
Others think this world overlaps with what has been called the third eye. Somehow this term persists even though we all know it is just an additional function at the center of the brain. In the past humans somehow thought for a while that seeing with the third eye was special. Luckily humans have come a far way and now all can perceive with that eye.
Two worlds. We can elaborate more on this. And that is for another day.