For thousands of years philosophers have gotten the science of our existence wrong. They have mistakenly assumed that we know or perceive the world around us; we do not, we cannot. They have believed that our sense perceptions are direct (or indirect) perceptions of the existence outside us; they are not, they cannot be.
Instead we are organisms that bump into the world—and the world bumps into us. For many of these bumps our bodies send electro-chemical signals to its brain. These electro-chemical signals are not sensations, they are not experienced. From them the brain selectively (usually but not necessarily) creates sensations which we call sense perceptions, but which in fact are simulacra—experiences created by the brain to stand in for the world around us (and in some cases stand in for our own bodies).
In some vertebrates, and certainly in mammals, the brain creates a visual simulacrum with which all other simulacra are more or less correlated or integrated. This integration creates the experience of a world around us. However, it is not the world around us: it is an assembled, (more or less) integrated simulacrum of the world around us.
We do not perceive and we do not know the world, we experience and know this simulacrum, created by the brain, which stands in for and represents the world.
The ability to create simulacra from bumps with the surrounding world is something that has evolved as organisms have evolved. This includes the evolution of simulacra which do not stand in for the outside world, but rather represent the organism’s aims or state of being in the form of urges, feelings, or emotions. These aid decision-making in a complex world.
In some species, simulacra have evolved which simulate the brain’s other simulacra; these are sensations which we experience as meanings, and which form the basis of language and cognition in humans (and perhaps in other mammals). In humans, the brain attempts to integrate these sensations of meaning into simulacra of knowledge, overall models which we can experience as understandings of the world and of ourselves.
Because we are continually bumping into the world, we are constantly in position to test these understandings of the world for reliability. In general—despite unfortunate exceptions—we strive to develop knowledge-simulacra which provide adequate (if not maximal) reliability regarding our interactions with the world and other organisms. More recently, the human pursuit of science involves using careful observation and deliberate experimentation to create models which have what Victor Stenger called point-of-view invariance. This scientific process has turned out to be extremely useful to human beings.
I have laid out a bit of the basic science of our existence. We need to understand these basics before we can build a coherent philosophical model which fits the world and our existence within it.
4-22-2016 > this was edited for clarity since its initial posting