The world as it appears to us is the result of how our senses perceive it. But that's not how it really is. The way we perceive the world is the result of millions of years of the evolution of our species. And species evolve so that they can better navigate their way through the world in which they find themselves.

If we had evolved on some other planet, we would have evolved differently. We would see things differently, everything in X-rays for example (if we needed to see X-rays in order to survive). Think of the parable of the blind men and the elephant, each man perceiving the creature differently. We are not blind, but the situation we find ourselves in is no different. In a very real sense, everything we perceive is in our head.

What, then, really is? The universe as revealed by scientific instruments? But which instruments? Telescopes can detect light at different wavelengths--optical, X-ray, gamma ray, ultraviolet--and all are equally valid. The universe is the sum of all possible perceptions.

To an individual species, however, all possible perceptions would appear as a muddled mess. A species doesn't need to experience all possible perceptions in order to navigate its way though the world. Looked at this way, one can think of a species as an entity defined by the set of perceptions passed through by its senses, a subset of all possible perceptions.

Qfwfq was right. The universe is nothing but a set of mathematical equations. And I am a footnote to a proof. A quod erat demonstrandum.