What is life?
Life is a collection chemical reactions in an energy gradient across a membrane. Fundamentally, what drives life is the reduction of CO₂ by hydrogen. This results in the production of complex carbon compounds, which I call macro-molecules. Life as we know it involves four main kinds of macro-molecules: proteins, nucleic acids, lipids, and organo-metallic complexes. We are ~90% water, and ~9.9% macro-molecules and ~0.1% salt of various kinds. Some of the macro-molecules, particularly the nucleic acids, have the ability to self-replicate. Self-replicating molecules function as structural elements, catalysts, or as templates for the production of the first two.
The original energy gradient was probably hydrogen and methane gas bubbling up from warm alkaline undersea vents into cold acidic, CO₂- and iron-rich sea water; through porous structures made of precipitated calcium-carbonate. Nowadays the most important energy gradient is provided by sunlight falling on the surface of the earth.
The dominant form of life is bacterial (taking in the Kingdoms of Bacteria and Archaea). It has been for at least 3.5 billion years and probably nothing will change that. Eukaryotes (all other forms of life) are certainly everywhere, and multi-cellular eukaryotes certainly make a lot of fuss, but bacterial life is more numerous, more diverse in form and genetic variation, adapted to a greater range of ecological niches, and greater in biomass. Furthermore, all other forms of life rely on bacterial symbionts to survive: from mitochondria and chloroplasts within animal and plant cells, to our gut microbiome. Without our bacteria symbionts, we'd be dead. And the next most dominant form of life are fungi. 99% of life on earth is bacterial or fungal. And 99% of eukaryote life is plants. 99% of animal life is invertebrate. 99% of vertebrates are fish. Whatever led humans to consider themselves the dominant life-form on the planet?
Life is intrinsically interesting because it is exceeding complex and self-sustaining. Life modifies the environment to make it more suitable for life, consuming resources and converting them into waste products, which in turn become resources for some other form of life. Life shifts the environment far from its natural (or chemical) equilibrium. Life is all interrelated and interactive. Everything relies on everything else.
In this age of individualism, winner takes all, and survival of the greediest, the fundamental themes of life—interconnectedness, communities, cooperation, symbiosis, ecological networks, recycling, equilibrium (or homoeostasis)—give us an alternative starting point for thinking about how we understand the world, our place it in, and how we ought to live. In the long term, our birth and death are simply short cycles of resources being used to create structures and then being returned to the pool for reuse. Ideally how we live will be conducive to life generally, but life is incredibly adaptive and it won't matter how we live in the long run: life will adapt. And when we die, our molecules and elements will be recycled just the same. We are waves in a field of resources; rising, falling, rising.
But what we mean by "life", in the context of life after death, is usually tangled up with notions of conscious life. When we talk about "life after death" we don't mean life per se. We don't mean the chemical reactions. We mean life in the sense our conscious existence. What we seek in talking about life after death is continuity of our inner lives. Life after death plays on the ambiguity of the word "life". To be more accurate we ought to say "consciousness after death", rather than "life after death".
Consciousness After Death
On one hand life does continue after death. Our bodies become food for a host of bacteria and fungi which recycle everything we are made of and return it to the environment to be used by other forms of life. Nothing is wasted in life. Indeed some people have observed that most of the molecules that make up our bodies all came from other living things originally. Even the air we breath is recycled. 99% of the oxygen in the atmosphere is excreted by plants and algae.
But this doesn't solve the problem of our attachment to our mental survival. Most people don't care about physically coming back after death, though zombies are a very popular meme at present. Most people accept that their bodies won't last, but want their memories, their personality, and their opinions to survive.
This way of thinking is only possible because we routinely divide the world into two: physical and mental. It is true that we know about the world in two main different ways, that for convenience we may label "physical" and "mental". But to generalise from this that there are two corresponding modes of being is a leap of faith. And it's not one that is supported by our systematic investigation of the nature of the world. Everything points to one mode of being, of which there are various manifestations and we which experience in a variety of ways because of the windows we have on the world, i.e. our senses.
The idea of a dual mode of being continues to appeal for a variety of reasons. Since our knowing seems to come in two varieties, the idea that the world is literally divided in this way seems plausible. There are a number of experiences, including dreams, sleep paralysis, out of body experiences, and so on that make a non-material mind seem highly plausible and almost certain to exist. We are predisposed to confirmation of our opinions and tend to stop seeking explanations once we have one that is even vaguely plausible. So dualism is the norm. It is wrong, but the reasons for this are subtle, complicated, and counter-intuitive. So it's hard to convince most people on this score.
Also we see the dissolution of bodies at death. The sights and smells of putrefaction elicit disgust because the by-products of this process are poisonous to us. Disgust protects us from eating poison. The naive view, however, sees the putrefaction of the body and rebels from the idea that the mind goes the same way. We deeply desire for our inner lives to continue. And the certain knowledge of death creates a cognitive dissonance.
So humans mostly create this split in their minds. They divide the world into physical and mental; or into matter and spirit. Each has strong associations and metaphorical entailments with the two modes of being. Physical is cold, hard, heavy, unresponsive, lifeless, typified by rock and by putrefaction. Mental is warm, soft, weightless, responsive, living, typified by light and renewal. Metaphorically spatial metaphors are important: we are standing, upright, and up when alive and well; prostrate, flat, down when asleep, ill, or dead. Physical is down; mental is up. Punishment is bad, hence down; hence Hell is the underworld. Reward is good, hence up; hence heaven is up. Matter is corrupt; spirit is pure. Matter is temporal; spirit is eternal. And so on. There is a net work of entailments and associations that make up a self-consistent worldview. It's just inconsistent with reality.
Dualism is False
Dualism is intuitive and its consequences are desirable. It implies that we can escape the fate of our bodies, escape putrefaction. We can in short survive death and live forever. Hallelujah. On its own this is too simple. But we are social animals, hierarchical, and moral. We are also biased towards perceiving things as conscious: animism is the most widespread belief there is. According to one survey I read, 100% of modern hunter-gatherers are animists, while only 80% also believe in an afterlife. If life really resides in the spirit side of things, the disembodied living things become plausible, and our bias towards seeing consciousness in the world reinforces this. So most pre-modern humans live in a double world: a world of matter, with beings made of matter but enlivened by spirit; and a world of pure spirit. Special people, shamans, can bridge the gap and communicate between worlds. In civilisation shamans become priests.
As intuitive and plausible as it seems, dualism is false. The two ways of knowing create the illusion of two kinds of world, but in reality they are two kinds of window on one world. All the reliable evidence we have about the world points to this conclusion. There is in fact no distinction between mental and physical being. We live in one world, at most. So none of the stories we tell that are based on duality are true: God, ghosts, spirits, the afterlife, ESP, rebirth, karma, etc. None of it is true. This is a tragedy. A wrench. A blow. A crisis. A source of cognitive dissonance. Most people will not accept this argument because it conflicts too much with what they think they know.
Even atheists often accept this intuitive dualism. Scientists tacitly accept the dual nature of the world even though they argue that only the material is real. You cannot have an argument over which are real (or more real)—mental phenomena or physical phenomena—unless you first accept that the distinction is valid. If like me, you reject this distinction, then the scientific materialism argument starts to look as suspicious as any religious argument.
Unfortunately, this insight into the true nature of the world, the one world, means that the life after death that we crave is not possible. This is because what we think of as mental is not separate from what we think of as physical. To put it another way, the part of the world that we view through the window we label "mental", is not different from the part of the world that we view through the window we label "physical".
A dramatic demonstration of the oneness of the world and the relation between mind and matter can be found in the aetiology of Alzheimer's Disease. In this disease, protein plaques form that disrupt the connections between neurons and eventual kill them, especially in the hippocampus where memories are made and stored. As the connections in the brain as disrupted the person progressively loses their ability to function in the world. New memories stop forming, then older memories are lost. One gradually loses the ability to recognise people, places, and things. One's sense of identity, which is based on memory, is degraded and gradually lost. Sufferers have increasing problems with reasoning, concentration, and orientation. Changes in personality such as aggressiveness may appear. Eventually a person with Alzheimer's loses the ability to do basic functions like eating, and they die.
The progressive destruction of the brain destroys everything about a person that makes them unique and special; it destroys everything it makes them a person. It destroys their inner life, their personality and their opinions. And it does all this before it kills them. Presuming they survive long enough for the disease to progress that far, the person is gone long before the body finally stops metabolising. A more tragic end for a person is difficult to imagine. If one believed in a God, one would be tempted to conclude that a God who included Alzheimer's in their creation was incorrigibly cruel.
Dualism would predict that a disease like Alzheimer's would have no significant effect on the mind, because the mind is not dependent on the brain. Monism, the one world theory, predicts exactly what we see. Modified versions of dualism exist which try to offer workarounds for cases like this, such as the brain as radio antenna theory, but these fail to explain other aspects of mind functioning. Monism is the only worldview which correctly predicts the effects of Alzheimer's.
One World, One Life.
The hard truth is that we only live once and we live that life in one world. But the way we evolved makes us susceptible to all kinds of belief about life and the world that are not true. So weirdly, most of us live out a delusion. And many people are happy to exploit that susceptibility to delusion for their benefit. Some even sincerely believe that their delusion is a better delusion than your delusion. The thing about genuine delusions is that they are compelling. A genuinely deluded person has no conception that they are deluded. They understand themselves to be seeing things as they are. Given that delusion is the norm, it's better to assume a sceptical stance and assume that one is not seeing things as they are. There is always room for improvement.
I tend to state things as I see them, since it is only by doing this that one can be clear about what one thinks at any given moment. But I think one can see that my thinking evolves over time. I'm prepared to accept new information and to change my mind.