At the physical level, life is manifest as a process. Every action of the purely physical (non-living) must be started and stopped in relationship to other physical existents (including internal components of machines, for example). The life process differentiates living organisms from the merely physical, because it is self-generated and self-sustained, that is, nothing outside the life process itself starts it or or sustains it.
Life is a quality that differentiates between those entities we call organisms, and all other entities. The self-sustained process, itself is performed by the organism and maintains the organism, as the kind of organism it is; so long as the process continues, the entity remains a living organism, but the moment the process ceases, the organism becomes a non-living (merely physical) entity.
The behavior of all entities, except living entities, can be described entirely in terms of their physical nature, that is, in terms of the laws of physics and chemistry. It is because the behavior of living organisms cannot be adequately and completely describe without reference to the life which differentiates them from other entities as organisms that they are ontologically different from the mere physical.
The popular physicalist view of life holds that it is just a very complex manifestation of physical qualities and might be described in those terms. An organism can be studied in terms of its physical/chemical/electrical behavior (as biology does), but such a study will not discover those aspects of an organism's behavior which is "living," because the behavior of a living organism, as an organism, requires life, which the physical/chemical/electrical nature alone does not describe or account for.
The differentiation of material existence by the quality "life" does not cause or allow any quality of physical existence to be violated. Life is an attribute of physical organisms and it will never be discovered within an organism that any physical laws are violated. But the physical laws do not cause, or give rise to, or explain life.
Life is not something "added" to or "injected" into an entity, it is a completely natural attribute, like any physical attribute, but which only organisms have as an attribute. Since no attribute can exist independently of the existent it is an attribute of, the life attribute does not exist except as an attribute of physical organisms.
The Unique Attributes of Life
Technically these attributes should be called attributes of organisms, because the only identifiable attribute of life is that it differentiates living organisms from mere physical existents. It is the life attribute, however, that makes the six unique attributes of living organisms possible. No physical property or action can produce or account for them. Those attributes are the following:
[NOTE: I use the word "material" to identify anything that exists independently of human consciousness or knowledge. "Independently" does not mean, "separated from," it means, "whether or not," anyone has any knowledge or consciousness of them. The material world is also called natural existence, or just nature. The physical is a subset of the material and includes everything human beings can be directly conscious of, and all that can be learned about that we are directly conscious of, but excludes life, consciousness, and the human mind, which exist materially, that is, naturally, but not physically. Life, consciousness, and the human mind only exist as attributes (or phenomena) of physical organism, but are not themselves physical attributes or explainable in terms of physical attributes.]
Self-initiated, self-sustained Process
Physical existence is that existence we directly perceive, the world we see, hear, feel, smell, and taste. We cannot directly perceive life, we can only observe the unique behavior of living organism for which life is the explanation, because it is behavior not possible to non-living entities and which cannot be described in physical terms.
All of the physical aspects of the process that is called life can be observed, but the attribute of life itself, which makes the process possible, cannot be directly observed, because it is not physical and has no perceivable qualities. It is this imperceivable aspect of life that makes it subject to mystical interpretations.
The existence of life cannot be detected by any physical means, because it has no physical attributes, that is, it has no mass, no temperature, no size, no shape, no color, no electrical or chemical properties. While an organism may have some or all of these attributes, it is the physical aspects of the organism as a physical entity, which every organism is, that has those properties, but it can have those properties whether it is living or not.
Life is a real material thing (because it exists independently of our consciousness of it), and a thing, the presence of which (in living organisms) and absence (in non-living entities) is apparent. But it is not, as the mystics and many philosophers mistakenly think, something added to the physical, it is a unique attribute of the physical which differentiates it into the living (when present) and non-living (when absent). It is not a substance (which would make it physical) and not supernatural (which would make it unknowable) but a perfectly natural thing, a real aspect of the material world.
I call it a thing, because it is certainly not nothing, but it is not an entity in any sense. Any other description would be dabbling in mystic conjecture. Nevertheless, organisms have attributes that only life makes possible and those attributes can be objectively identified and described. The remaining attributes in the above list are those attributes. [See life is a thing, below.]
[NOTE: If life cannot be perceived, how can it be known? By, "physically undetectable," is meant life itself, the attribute, cannot be directly or indirectly perceived. The fact of life is axiomatic in all living organisms. Life does not need to be perceived to be known, first because it is axiomatic, and secondly because we ourselves are alive, which we know in the same way we know we are conscious, not because we can prove it, but because we are.]
Self-initiated, Self-sustained Process
Life as a process is both self-initiated and self-sustained. Nothing outside a life process starts it, and nothing except the process sustains it.
From a physical perspective, only the physical aspects of that process can be directly observed, that is, science can only deal with those aspects of living organisms that are physical. The sciences that study the physical aspects of an organism are biology, anatomy, neurology, and biochemistry. What science cannot study, are those aspects of the life process that are not physical, which are the attributes unique to living organisms because of their life, not their physical characteristics, which cannot give rise to them.
[Note: By a process I mean a continuous action comprised of any number of related actions which produces a particular effect or product. The life process is a self-caused process with itself as the product. The initiator of life is life and the product of life is life. The living process is the only process which produces itself.]
The true nature of cause is not, "same cause same effect," but the nature of the entities that act. The use of the word cause ("self-caused process") is justified here in the colloquial sense, that is, as "the reason" for something. Life is the reason for the life process and the life process is the reason (by means of sustaining it) the life.
At the physical level life manifests itself as a process that is self-sustained, and determines the behavior of an organism as a living entity. The living behavior of an organism cannot be understood or explained apart from the life process.
Non-living physical things do not sustain themselves. They may remain for long periods of time as the kind of things they are if substantial enough (like a diamond) or quickly change into something else (like a drop of water). But no physical thing acts to maintain itself as the kind of thing it is. All its behavior can be explained entirely in terms of physics and chemistry. An organism's behavior, as an organism, must sustain itself as the kind of organism it is, or it ceases to exist.
It is very easy to mistake the essential nature of an organism as, "purpose." such as, the "purpose of an organism is to sustain itself as the kind of organism it is." Except for human beings, an organism has no, "purposes;" its living behavior does sustain itself as the kind of organism it is, but it does not do that because it is its purpose, it can do nothing else.
If an organism did not sustain itself, it would simply cease to be. To call what any natural thing does (with the exception of human beings) a purpose is a kind of anthropomorphism, attributing human traits to non-human existents.
For merely physical events, there is no connection between events and the consequences of those events, except the physical events themselves. For living behavior, however, both the behavior and the consequence of the behavior (what it produces) are the same thing, the life of the organism. There can be no physical explanation for why anything's behavior should be to sustain itself, which means, there is no physical reason (explanation) for life.
My meaning for sentience is somewhat different from the meaning usually attributed to it. Sentience, pertaining to the self-sustaining life process of an organism, refers to an organism's response to external stimuli which is dependent on the life process.
A "response" to stimuli is not the same as a non-living physical "reaction" to an external influence. A container of water might react to an impact or sound waves impinging on it, but that reaction is entirely physical and totally explainable in terms of physics. The "response" of a living organism to outside influences called stimuli, is an action made possible and required by the life "process" of the organism. If for any reason, the life process should cease, that response to stimuli would cease, even though all the physical attributes of the entity remain the same. It is the process itself that reacts to the stimuli, indicating the process detects the presence and nature of the stimuli in order to react to it.
If an organism could not detect a stimulus, it could not react to it. If an organism could not distinguish the differences in stimuli, it would react in the same way to all stimuli, or react randomly without any connection between the nature of the stimuli and the action. This is what distinguishes a physical reaction from a living response. A response is the result of the organism in some way detecting (not identifying) the presence and nature of the stimuli, a reaction is an immediate action attributable directly to the external influence and laws of physics (even if the reaction is a very complex one involving a computer program, for example).
The particular things an organism will react to and the specific response the organism makes is determined by the organism's nature as an organism, that is. If the life process ceases, the organism reverts to being a mere physical entity, and its behavior reverts to that of any other non-living entity, including its reactions to external influences.
By unity I mean for every organism there is only one life and it is that life that is responsible for (makes possible) all the living behavior of the organism, and that life remains uninterrupted from the moment the organism begins living until the organism ceases to live. (This is true even when an organism is in a state of stasis or suspended animation—a seed for example.)
By continuity I mean an organism's life is the same life from the moment it begins to exist as an organism until the moment it ceases to exist as an organism.
Both unity and continuity are attributes of consciousness as well, and much more important, philosophically, there. Please see the chapter, "Consciousness."
Life, A Thing
Life is an existent, a thing, but it is not physical, and therefore must not be thought of as an entity, or a substance, but an existent uniquely identified by its attributes or qualities. Everything that exists is identified by its attributes and qualities. Physical things are identified by their physical qualities, such as size, weight, color, shape, or temperature. Life is not physical, and therefore has no physical attributes or qualities.
The attributes of life are those just discussed, that which differentiates living organisms from non-living physical entities which manifests itself as a self-sustained process that is responsible for an organism's sentience and living behavior.
Life is not physical, but it cannot exist independently of a physical organism of which it is the life. It is not supernatural, but a perfectly natural aspect of reality, the aspect that is the life of all living organisms.
As a thing, life is not something "added to" the physical, like a spirit entering into non-living matter, giving it life. It is very much the opposite, in fact. Life always takes on non-living matter and makes it living by making it part of the organism it is the life of. Life grows, not by moving out into more and more non-living matter, but by including more and more non-living matter in itself.
Life and Science
As far as we know, life only comes from life, and the growth and expansion of life has only one source and cause, which is life itself. Modern science assumes it is possible for life to spring up from non-life (abiogenesis), if the conditions are right, though no one can say what those conditions must be, and no one has ever observed such an event. There is really no "scientific" basis for this view, except for an extreme dislike for the idea that life can only come from life, (biogenesis), and because it makes the prevailing evolutionary hypothesis less than scientific. If science, as well as philosophy, are to be based on observation, there is no observational evidence that life has ever or can possibly come from non-life.
[NOTE: Philosophy does not reject speculation about such things as origins or even the origins of life, so long as such speculation is not called science or philosophy. There is also nothing in science or philosophy that rejects the possibility of life on this planet having an origin so long as that origin does not suggest that life "emerged" from the physical. Life is not a physical attribute and no arrangement or organization of the physical can produce life. If life, in the form of the organisms found on earth, began here, life would have to have existed in some manner as an essential potential attribute of material existence before being manifest as recognizable organisms. I do not speculate this is so, only that there cannot be real existence void of life.]
It is not within the province of philosophy to speculate about the origins of things, or even if there are origins. (It is philosophically correct to deny the possibility of nothingness.) It is the purpose of philosophy to discover the nature of what is, not how it got here.
[NOTE: I've been told one reason to study origins is to help understand how things are now. That might be if we had a record of how things got here first, like the auto-mechanic asking the customer, complaining of a rattle, if he has hit any really bad bumps recently. For those interested in how things got here, there is only what is here to go on. First, what is here must be understood and perhaps some things might be discovered from that about how they got here. Trying to figure out the nature of things that are here, from how they got here, is simply backwards.]
[NOTE: The evolutionary hypothesis has many philosophical and scientific problems that are seldom recognized, much less addressed. Whether those problems are ever resolved or not, the hypothesis certainly should not be used as the basis for assertions about the nature of man, the nature of life, aesthetics, ethics, or social values. Doubts about evolution, however, are certainly not grounds for speculation about some supernatural or mystical views of origins or anything else.]
In all of my discussion of the nature of life, there is still something left out: the profound difference between living organisms and mere non-living matter, which poets seem to notice, and some philosophers, but not so many scientists, although some in the field of medicine certainly do.
In this universe there is life, even if the only life in it is on this tiny speck in the universe we call earth. Without that life we would not be either. Existence is not contingent, so the universe is what it is, and could not have been anything else. We might be able to imagine a universe without life, but what would that universe be? If there were such a thing, it certainly wouldn't matter—nothing matters if there is no life, because there is nothing to which it could possibly matter.
The break between the merely physical and the living is indeed profound, in ways perhaps only a poet can rightly express, but this poor philosopher at least recognizes that, as astounding as the physical universe in all its grandeur is, without life it is a pointless show in an empty theater. But the universe does have a point, because there is life in it, and it is astounding, because we are in it to be astounded by it.
The physical can be studied entirely in terms of the physical sciences, and in those sciences, philosophy has no business intruding, as long as no supposed science attempts to contradict any philosophical truth. There can be no contradiction between science and philosophy. At any point if a contradiction arises, either science is wrong, philosophy is wrong, or both are wrong.
I think some philosophers worry about the possibility of any aspect of reality being non-physical, that it would somehow make all our knowledge unreliable. After all, science has been so successful, and the evidence of it is in everything that exists in the modern world that has so improved our lives. If anyone really has that worry, consider this: it is true, living organisms are not predictable in the same way as the rest of physical existence, because what they do is determined by things science cannot study: life and consciousness. Beyond that human beings are even less predictable because they have volition. But in the entire universe, as far as we know, the number of these unpredictable living things is infinitesimal compared to the rest of existence. I deny that the nature of living things cannot be understood as well as the physical, but even if they could not, in the grand scheme of things, except for the tiny number of living things, our knowledge of the predictable physical universe is perfectly safe.