We can now attempt to define the underlying factors that guide societies in determining what is real and what is fantasy, not only in the physical world, but in our own, human-created, memetic universe. And it doesn't give a damn whether we understand it completely, or only partially, or not at all. The history of human evolution, since we attained our present physiological form, is essentially the history of our efforts to understand that real world better and figure out ways to bend it to our purposes. When we achieve a new level of understanding we adapt to and use the outside world more effectively. Our understanding achieves a higher level of functionality.
Functionality has to be a key element in the world of the mind as well as in the physical world.
The two do not exist independently, as Descartes suggested. On the contrary, the interaction between them is continuous and intimate. A culturally specific idea or technique that enhances a group's prospects for physical survival in competition with another group is likely to be a winner, and the culture of the first group will soon come to take for granted that it is correct behavior. Or in other words, that it is valid, and that alternatives are not. From there it is but a short step to assume that it is part of the reality of their way of life, as opposed to the unrealities, or wrongness, of unproven alternatives.