What is the Underlying Nature of Reality?

The basic starting point is that there appears to be a universe that is external to our minds and that this universe exhibits the properties of stability, consistency, structure, and causality. All of this can be explained in the following manner.

1. Reality is external to our mind, and we have a body that is part of this shared physical universe.
   1. This is the concept that the universe is made up of matter and energy and that their relationship and how they work are described as using scientific tools and methodology. {This is the belief that I accept as valid.]
   2. Plato's allegory of the cave is a variant of this since, according to the allegory, we are physical beings who are chained to seats in a cave with other physical beings watching a shadow show projected onto the cave wall. We are so intent on watching this play of shadows that our immediate experience of the word is of the shadow performance. Plato's point is that we are not waking up to the fact that we are actually in a cave and could leave the cave to enter the upper realm of the physically larger and real world. The Matrix movie series is a modern variation on Plato's theme. The bottom line is that there is a shared physical universe in Plato's cave and in the Matrix; it is just that there is confusion about what is real and where is it that our physical bodies are located.
2. Bishop Berkeley's Idealistic Cosmos: We are just immaterial beings—minds/souls/spirits—sustained in the reality created by God.

[Emphasis added]
In his two great works of metaphysics, Berkeley defends idealism by attacking the materialist alternative. What exactly is the doctrine that he's attacking? Readers should first note that "materialism" is here used to mean "the doctrine that material things exist". This is in contrast with another use, more standard in contemporary discussions, according to which materialism is the doctrine that only material things exist. Berkeley contends that no material things exist, not just that some immaterial things exist. Thus, he attacks Cartesian and Lockean dualism, not just the considerably less popular (in Berkeley's time) view, held by Hobbes, that only material things exist. But what exactly is a material thing? Interestingly, part of Berkeley's attack on matter is to argue that this question cannot be satisfactorily answered by the materialists, that they cannot characterize their supposed material things. However, an answer that captures what exactly it is that Berkeley rejects is that material things are mind-independent things or substances. And a mind-independent thing is something whose existence is not dependent on thinking/perceiving things, and thus would exist whether or not any thinking things (minds) existed. Berkeley holds that there are no such mind-independent things, that, in the famous phrase, esse est percipi (aut percipere) — to be is to be perceived (or to perceive).
[https://plato.stanford.edu/entries/berkeley/#3.2.1 ]
 
3.1.3 God's existence
The last major item in Berkeley's ontology is God, himself a spirit, but an infinite one. Berkeley believes that once he has established idealism, he has a novel and convincing argument for God's existence as the cause of our sensory ideas. He argues by elimination: What could cause my sensory ideas? Candidate causes, supposing that Berkeley has already established that matter doesn't exist, are (1) other ideas, (2) myself, or (3) some other spirit. Berkeley eliminates the first option with the following argument (PHK 25):
 
(1) Ideas are manifestly passive—no power or activity is perceived in them.
(2) But because of the mind-dependent status of ideas, they cannot have any characteristics which they are not perceived to have.
 
Therefore,
 
(3) Ideas are passive, that is, they possess no causal power.
 
It should be noted that premise (2) is rather strong; Phillip Cummins (1990) identifies it as Berkeley's "manifest qualities thesis" and argues that it commits Berkeley to the view that ideas are radically and completely dependent on perceivers in the way that sensations of pleasure and pain are typically taken to be.
 
The second option is eliminated with the observation that although I clearly can cause some ideas at will (e.g. ideas of imagination), sensory ideas are involuntary; they present themselves whether I wish to perceive them or not and I cannot control their content. The hidden assumption here is that any causing the mind does must be done by willing and such willing must be accessible to consciousness. Berkeley is hardly alone in presupposing this model of the mental; Descartes, for example, makes a similar set of assumptions.
 
This leaves us, then, with the third option: my sensory ideas must be caused by some other spirit. Berkeley thinks that when we consider the stunning complexity and systematicity of our sensory ideas, we must conclude that the spirit in question is wise and benevolent beyond measure, that, in short, he is God.
 
3.2.2 Hidden structures and internal mechanisms
The related notions of regularity and of the laws of nature are central to the workability of Berkeley's idealism. They allow him to respond to the following objection, put forward in PHK 60:
… Berkeley's answer, for which he is indebted to Malebranche, is that, although God could make a watch run (that is, produce in us ideas of a watch running) without the watch having any internal mechanism (that is, without it being the case that, were we to open the watch, we would have ideas of an internal mechanism), he cannot do so if he is to act in accordance with the laws of nature, which he has established for our benefit, to make the world regular and predictable. Thus, whenever we have ideas of a working watch, we will find that if we open it, we will see (have ideas of) an appropriate internal mechanism. Likewise, when we have ideas of a living tulip, we will find that if we pull it apart, we will observe the usual internal structure of such plants, with the same transport tissues, reproductive parts, etc.
[Downing, Lisa, "George Berkeley", The Stanford Encyclopedia of Philosophy (Fall 2021 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/fall2021/entries/berkeley/.]
 
Therefore, in Berkeley's world, it appears external to our minds and functions exactly as science describes it. The only addition to this is that Berkely presupposes that the underlying reality is that despite how things appear and seem to work, the world is merely an idea created and sustained within the mind of God, including our mental existence.

Solipsism is a variant of those two choices. 

Solipsism could be a variant of Choice 2a. It could be the case that you are the only immaterial being, aka the solipsist is God, and you imagine everyone and everything else—hence this is just a version of choice #2.
Or: you realize that a mind does not exist outside of a body, and thus though you might be the only mind that you are immediately aware of, you have a physical body thus in either some form of Choice #1a—a variation of Plato's Cave allegory and there is a physical shared world or Choice #1b: you are in a coma/asleep, and you are currently dreaming, and when you wake up you will find ourselves in a shared physical world, and that is just a version of choice #1.

Conclusions:
It is noteworthy that as a child, we only first experience and consider the world according to choice #1. 

All those other choices only come to us later in life if we are taught them directly and completely and thereby convinced that the idea is true, or we are taught to doubt the reality of choice #1, and we derive and invent one of those other choices on our own.

If you believe in Choice 1a or 1b or 2 or 2a, there is nothing I can say that will completely and utterly convince you that your idea of the nature of reality is false. You can even dismiss my noteworthy statement that you had to be taught to doubt the original premise of choice #1. Nor if you believe in choice 1a or 1b or 2 or 2a could you possibly convince me that choice #1 is false. Hence we are at one of James's pragmatic impasses.

According to James's Pragmatism, an idea has to make a difference; if it doesn't, it can be ignored. James explained it this way:
The pragmatic method is primarily a method of settling metaphysical disputes that otherwise might be interminable.  …The pragmatic method in such cases is to try to interpret each notion by tracing its respective practical consequences. What difference would it practically make to anyone if this notion rather than that notion were true? If no practical difference can be traced, then the alternatives mean practically the same thing, and all dispute is idle.[1]
 
It is astonishing to see how many philosophical disputes collapse into insignificance the moment you subject them to this simple test of tracing a concrete consequence. There can be no difference anywhere that doesn't make a difference elsewhere—no difference in abstract truth that doesn't express itself in a difference in concrete fact and in some consequent upon that fact, imposed on somebody, somehow, somewhere and somewhen. The whole function of philosophy ought to be to find out what definite difference it will make to you and me, at definite instants of our life, if this world-formula or that world-formula be the true one.[2]
 
Therefore, to restate the point, in the end, it doesn't matter which is the 'True' underlying nature of reality amongst those choices since in all of them, we experience reality as if there is a shared physical world that we inhabit with other physical beings. Solipsism is irrelevant as it is merely a variant of Idealism or the Plato Cave/Matrix. There is no difference in how the world functions and how we can discover and describe how the world functions between Berkeley's Idealism, the Plato Cave/Matrix, and the acceptance that the world is made up of matter and energy. In each of these, we still can use the tools of science to attempt to describe accurately how things work, and it does indeed succeed in that process. Therefore, we need not concern ourselves with the only real challenge to accepting that the world is material and external to our mind that Berkely offers since the idea that everything exists in God's mind makes no difference to scientific exploration and explanation of the universe's workings.


[1] (W. James, Pragmatism: A New Name for Some Old Ways of Thinking 1907, 28)

[2] (W. James, Pragmatism: A New Name for Some Old Ways of Thinking 1907, 30)