The first four philosophy posts were:
The bottom line is that I do not exactly know the world as it is. I know the world as my mind processes it. I know the world as it appears to me. I know the world from a finite, inevitably somewhat skewed perspective. 
One way that Kant put this conclusion is that I cannot know the "thing-in-itself" (das Ding an sich). I cannot know a tree apart from the way my mind processes the sensory information that comes into my mind through my senses. The work in which he especially develops this situation is called, A Prolegomena to Any Future Metaphysics.
The title suggests, more or less, that metaphysics is now dead. We have no point of reference to know what the nature of reality is. We know how our minds process it, but we cannot see the world in an unfiltered way.
2. It is not entirely clear then what we should do with earlier debates over whether the universe was made up of ideas, matter, or both. It's not clear how we would ever answer such a question.
So materialists would argue that everything that exists is made up of matter. Epicurus, Lucretius, these are famous ancient materialists. Of course many individuals in the modern era have more or less been materialists in the philosophical sense, believing that nothing but what we see exists, matter.
There have also been idealists, who believe that everything that exists is basically idea. Perhaps the most bizarre, to us moderns, was Bishop George Berkeley (1685-1753). He believed that we are all the thoughts of God, that "to be is to be perceived" by God, to be an idea that God has.
Of course, how would we tell the difference? We look around and see stuff. But we don't see what it is. How would we tell the difference between a "material" atom and an atom that was an idea in the mind of God? So the pure material versus pure idea debate seems impossible to resolve, perhaps even senseless.
3. On a popular level, most Christians are probably dualists, someone who believes that the universe is made up of two types of stuff, matter and say spirit. You might think of a person as a body and a soul. It is true that we cannot observe a soul, at least not from the perspective of our senses. No credible experiment has been able to detect such a part of us. For materialists, that is disproof enough.
However, dualists would not usually think of the soul or spirit as something that could be detected by our senses. Since the scientific revolution of the 1600s, it has been typical to think of the "natural" as that part of reality that we can explore with our senses by discovery. Then the "super" natural was understood to be that part of reality that is "above" or beyond that which we can see with our eyes.  Descartes thought of the spirit realm as "immaterial" in a way that was new.
Again, these are scarcely things that we can prove or know at this time, that is, of what "stuff" these things might consist. The Bible presents the nature of reality in the categories of its day. The Old Testament presents reality as the Semites thought of it. The New Testament presents reality at times the way the Greeks did.
These are different ways of picturing reality. They are not separate from what other people thought in the ancient world. They are not some distinct biblical worldview. They are the Greek and Semite worldviews, the truth of the Bible incarnated in the categories of the day.
So, as we have argued of science in general, these are pictures of the world that "work." When we say that we as humans have a body and soul or a body and spirit, we are saying that we will exist after death. The point is that those in Christ will exist forever. The point is that we will be conscious between our death and resurrection. The point is that those who are not in Christ will face a judgment after death. The point is not the literal form we will have when any of these things take place.
So there are Christians who are "non-reductive physicalists." They believe that all human existence is embodied, that no one can exist in a detached, spiritual form. They believe in resurrection. They may believe that we will have some sort of body between death and resurrection. In short, they believe all the essential orthodox things to believe. They do not believe that mere materiality is all there is. They believe in God and (embodied) angels.
And again, we have no way to know one way or another. It is a speculative question. We know the point. We don't know the literal nature of the reality behind the point.
4. The physics of the atom has created great uncertainty even for the materialist as to what material reality is. The world smaller than the atom would seem to be a fuzzy world of probability and uncertainty. Gone is the determinism of the 1600s to the 1800s, where many thought that the future was entirely a matter of following the equations out forever. Determinism means that the future is already determined, simply playing out the laws of cause and effect.
But now it seems that nuclear particles may not actually have fixed locations, speeds, spins, and so forth at all. Not determinism but uncertainty and indeterminism seem to characterize the subatomic world. There are still many questions about this most fundamental part of the material world, so we should not draw any final conclusions. But it would seem that material reality on its most basic level is far from solid.
5. With classical metaphysics more or less dead, there were some philosophers in the twentieth century who spoke of other kinds of "being" or existence. We will talk more about "existentialists" in the next post. They held that we create who we are, as it were, by defining ourselves, choosing a life.
For now we want to mention those we might call phenomenologists. If I am speaking "phenomenologically," then I am speaking as things appear to me. If I say that I saw the "sun rise," you know I am not saying that the sun goes round the earth. I am simply speaking in terms of how the sun appears. I know that, literally, the earth is rotating and has turned such that I can see the sun and it appears that the sun has risen.
This talking of things as they appear has allowed us to speak of "being" in a new way. Martin Heidegger (1889-1976), for example, spoke of "being there" (Dasein), of who and what I am right here and now. He did not mean who I am as atoms or material or ideas. He was building on the existentialist idea that we decide who we are. If we embrace who we are now, then we have an authentic existence. Reality becomes a personal thing rather than an objective, outside of ourselves thing.
6. So we return to the pragmatic realism and critical realism of an earlier post. It would be absurd and completely useless to live as if the world around us did not exist.  Belief that the world around us exists "works" (pragmatic realism). By faith we can go further. By faith we believe that the world outside us exists. As Christians we believe that God exists and that he created the world, even if our apprehension of the world is finite and inevitably skewed (critical realism).
We do not need to know what is behind the metaphysical curtain. In the words of Ludwig Wittgenstein (1889-1951), "Whereof we cannot speak, we must be silent."
Next week: Philosophy 7: What is a person?
- Plato's Republic
- Immanuel Kant, Prolegomena to Any Future Metaphysics
 This was a paradigm shift in itself. Prior to the scientific revolution, reality was more seen as a continuum, a chain of being from God down to minerals. Angels were material, only much thinner material. But after the scientific revolution, God and spiritual beings are conceived as "immaterial," of a different kind of being altogether.
 Solipsism is the hypothetical perspective that only I exist. Only a truly bizarre or unhinged person could hold it.