The thing is that the argument is necessarily simple, because it rests on fundamental reality.
If you start from skepticism, which everyone should, you have no knowledge or beliefs. It's a blank void to fill. Your next choice is to devise some coherent method to create truth, or to simply leave it up to capricious nothingness and think no further on it. It could be argued that extremely unphilosophical people stop at this stage. They just never follow the path of thought and allow things to play out. Fine. One cannot argue with anyone's choice at this first stage, because any critique will rely on belief, of which we have none so far.
But most of us who are thinking about it, and therefore I wager all of us reading a thread like this, seek consistency and truth. This is in a sense the first step - an arbitrary decision, a perfect leap of faith, to seek truth and and reason.
I would say that the consistent, rational thing to do, faced with this blank void, is to start off by examining your impressions, or those things which present themselves as true before you, and then accepting them in order of their strength.
This of course should remain uncontroversial with all of us. All I've said so far is "nothing is certain, but we must believe what appear to be true to us."
But it's important to remember that this is where are coming from. We start there, not with the Big Bang, not with natural selection, not with our birth even. It is an immediate process contained completely in the present, actual, real moment. There is nowhere else to start, unless you reject the initial claim of accepting appearance. Certainly a man is entitled to the belief that time runs backwards, but we would not call it rational or pragmatic. Hume said "philosophy would render us entirely Pyrrhonian, were not nature to strong for it."
Anyway, so on to the impressions. What comes first? I doubt we will reach hard and fast and universal agreement about the exact order of magnitude in which they appear, but some common themes should emerge. First out of the gate are some basics, that come even before sensory perception, or perhaps before is a poor choice of words - they come in stronger than the sense perceptions.
These are simple things. Selfhood and agency - which appears through a thought life. If you deny that this is an intuitive sense, you're deluding yourself. Children as young as four insist in studies that when a researcher does one thing, he or she *could* have done something else. We haven't gotten to neurons yet, so hold your horses.
Another one is the intuition of consistency, which we could say was first before anything, since it led us from the void to this inquiry in the first place. I call it the I stick to induction - the idea that there are ideas.
And a third is the existence of Nature. There are things that appear outside of ourselves, these come in through sense perception, very broadly defined. You look down as see and feel your body, you smell smoke on the wind. These appear outside of the self.
And last (again I am not trying to push a distinct order for these. For now it's enough to say they all come at once) and probably most contentious, I see an appearance or intuition or purpose or value. There is a sense of a why. We feel we exist, but not simply that, also that we must exist for a reason.
So there you have it. We start from these. A corollary about the observation of nature is a belief that other intelligent beings such as ourselves exist out there as well - it's important not to forget that lol.
So already we can reject any pure monism - solipsism and pure pantheism, as well as modern scientific materialism, are sophistry. You cannot refute the foundation of your thought system using your thought system. Well you can, but you can longer claim to be rational.
Example: nihilists, or, more simply, those who reject my fourth appearance: meaning. On what basis do they make their argument? On what basis can any argument be made in the absence of value and meaning? The implication is that they appeal to Truth, that they have seen the Truth, and valuing that over Value or Love, they are prepared to believe that this truth is that there is no meaning in anything. And yet this is an internal self-refutation. What sense does it make to say you value truth more than you value valuation itself? This is nonsense, just as the absolute rule "no absolute truth exists" is nonsense.
Similarly, if the progression of scientific inquiry, which is based on the intuition of natural law, begins to seem to suggest for instance that there is no self, this is a sign that you may have lost your way. None of those our initial appearance can be used with any legitimacy to undermine another, because none has any authority for that.
You would use Truth to undermine value and ethics? You forget that you must value truth to do this. You would use your understanding of Nature's existence to undermine your own existence (materialism)? You forget that you must exist yourself to understand nature. You would use your own existence to undermine that of Nature's (solipsism)? You forget that without Nature, you cannot exist.
One major problem with materialism, besides this ignorance of their starting point, is that they nothing of consequence to say on the question of Being, which is foolishness, because this is the first and most important question, since of course existence is a prerequisite for anything else. Materialists simply ignore the question.
What is painful about this is that the intellectual distance between a Dawkins-type and an Emerson-type is so small. People like me don't claim to know what caused the Big Bag - it is a great mystery. Neither does Dawkins, but he is uninterested in the question precisely because it is a mystery. He "solves" it in a most unedifying way - my dismissing its importance. I can almost hear his insufferable voice uttering "who cares what caused the Big Bang? All that matters is that it happened."
And yet this is a case where the question is more important that the answer. The answer to "why does something exist rather than nothing" runs deep. Existence is a miracle and a mystery, and yet it is the source of everything we love.
Similarly, our own existence as being ourselves is a mystery. If scientists ever claim they have "solved" it, it will invariably be in the way Dawkins "solves" the Big Bang - by ignoring it.
Yes, our brains, the organ in your skull, profoundly affects your thoughts, and is composed of neurons. But this is why those quotes from Emerson were relevant, dirge. That's great to know, but it changes nothing. We could completely and fully map a human brain, it would change nothing, because you have failed to answer the hard problem of consciousness, which is that you cannot explain how it is that I exist in a way a rock does not. A rock is just a department of the broader universe. Indeed my body is the same way. But I am not. I exist inside this universe yet separate from it, because I have a subjective experience and a perspective. Such a thing cannot be explained anymore than the origin of the natural laws can be explained.
If you would cry "God of the gaps" I invite you to fill these gaps, indeed, even to suggest how they might be filled. Even if you answer more questions on the topic, these large questions of being will remain unanswered, because they precede all else.
And so that is the core science lacks. Improved means to an unimproved end. We have a science of psychology that helps us predict how bipolar people behave, we have pills that calm us and inspire us, we have observations of the birth of galaxies and records of thousands of alien planets, but none of this, alone, does anything to ameliorate the condition of the soul, that is, to advance virtue, or any fundamental truth, because truth, without the meaning of truth, is dead.
This is the core of the poem "when I heard the learn'd astronomer." There is nothing wrong with pursuing science of the stars to learn of our place in this Universe. But to learn all that and forget why you wanted to know is true folly.
And on that last count I may add one last point: it is an incredibly stupid and irrational position that many scientific materialists now take that the magnitude of the Universe suggests our unimportance and accidental nature. This is madness. It is a base human error to believe that bigger is better. We are physically small, but without us to realize how small we are, there is nothing at all. A scientist with his electron microscope ought to know better than to claim that a small thing is an insignificant thing. Especially with fermi's paradox in mind, our existence may yet proved significant indeed.