What is real? This question is the most fundamental question, but also the most human. It is impossible, indeed laughable, to imagine another species asking this question. Indeed, the answer to that question has not only shaped human thought through the millennia, but has formed human cultures. Here is the briefest and most general outline of how mankind has answered that question.
Let us begin with the beginnings of written language, around 2000 B.C. We start here not because human thought started here, but because it is only the invention of writing that allows us to know what people of the past believed. The people of this time apparently universally believed in what today we would call paganism or polytheism. That is, they believed in a multitude of gods who had influence on the various aspects of the world and life. This polytheism took various forms, from the highly structured temple worship of the Egyptians to the more confused and primal idolatry of the bush tribe. However, at least two things are common. First was the recognition that the visible world was influenced by the invisible world (or the spirit world). The sea was not simply water and salt, but the realm of Neptune, or even Neptune himself. The second was the assumption or belief that the gods, however defined, were not separate from the world or the cosmos, but within it. Zeus or Ra are powerful, perhaps even immortal; but they exist as part of the furniture of the cosmos. This limitation, and their foibles, show that they are more like exalted men than truly divine beings. So the first answer to the question of what is real was that reality consisted of the world around us, which has both physical and spiritual parts.
This answer held sway for untold centuries. It only gave way when people and cultures began inspecting it more fully. How exactly do the physical (or visible) parts of life interact with the spiritual (or invisible)? More importantly, which aspect, the physical or the spiritual, is more foundational? Which is the ultimate reality? Three distinct answers arose to this question, and those three answers have determined, and are still determining, the shape of human culture and the flow of human history.
One answer is to assert that what is primary is spiritual, but not personal. This is the answer of the East. The great Oriental religions differ on many points, but agree that the physical world is an illusion. The term used to describe this is maya, which means deception. The physical world is maya, an illusion or trick. The goal of enlightenment is to rise above this illusion, and see clearly. This is the only escape from suffering. Yes, eastern religion (at least in its popular forms) contains a multitude of gods, but these are all subsumed under the idea of monism: all is one. Distinctions of any kind are maya. The final reality can be called Brahman, the One, (as in Hinduism) or the Void (Buddhism). In either case, the final reality is not a person, nor is it physical.
Another answer to the question of what is real is supplied by atheism, which can also be called materialism (only the material universe is real) or naturalism (only nature is real). Atheism, of course, is the belief that there is no God. So of necessity the atheist, when asked, what is real, gives the answer: only the physical universe. There is nothing outside of it (no supernaturalism) and there is nothing inside it that can properly be called “spiritual”. Everything is matter. Nothing but matter exists. Sometimes this matter takes the form of energy, so it perhaps is best to speak of reality as “molecules and motion”. Ideas of God or gods, life after death, and religion are only projections of the brain, caused by unguided evolution, or perhaps the result of a virus.
Perhaps atheism seems the opposite of eastern thought. In fact, they share one great tenet: reality is not personal; there is no Person who transcends the universe or created it with a purpose. Because of this, the ideas of right and wrong become problematic for both the Hindu and the atheist (with some of the most devoted and consistent adherents of each denying the very concepts of an objective “good” or “evil”).
The last answer to the question of what is real is given by the theist: reality is God and His created universe. The reality we experience is both physical (because it is created as such) and spiritual (because it is created by Spirit, that is, a non-physical person). Of course, in one sense God is primary over His creation: He can exist without it; while it cannot exist without Him (it is contingent upon Him). He is ultimate. But creation is real (not an illusion) since He wills it into being.
This answer is given by the three great theistic religions: Judaism, Christianity and Islam. Of course, the difference between them here is whether God is a trinity (three persons in one essence) or not (one person with one essence). Without debating which is true at this point, allow me to point out one great difference that results from believing in the trinity. If God is not a trinity but a single person of one essence, then love is not ultimate. It was created (or developed) when God created the universe for before then (or apart from His creation) there was no-one and nothing outside of the solitary God. The concept of love would have no meaning apart from creation. But if God is a trinity, then we have another possibility: that love is fundamental and eternal, and exists apart from creation. We even have the possibility that love itself is the reason for creation: not simply love for the beings of creation (though that is just as real) but the love of the members of the trinity for each other. This idea, it seems, underlies the affirmation of the New Testament that God is love, as well as the breathtaking thought that believers in Christ are actually invited to share in this inter-Trinitarian love (see John 17:20-26).
This, then, is the basis for the three great modern worldviews: eastern monism, naturalism, and theism. Though each has various branches, every branch arises from one of these three trunks. And each begins with their answer to the most basic question imaginable: what is real?