The Nature Of Life
Life is an attribute or quality of material existence, just as all the physical qualities are. All that exists are physical entities, the characteristic and nature of which are studied and described by the physical sciences in terms of those qualities of existence we call physical. A very small number of physical entities have an additional attribute to all the physical attributes called life. Physical entities with the additional quality, life, are called organisms. The life quality itself is not a physical attribute and therefore cannot be directly perceived as all physical qualities can by being seen, heard, felt, smelled, or tasted, and have no physical characteristics that can describe it, that is it cannot be weighed, measured, or detected by any means physical attributes are detected.
Nevertheless, life is a perfectly natural quality completely compatible with all the other physical attributes of an organism and is dependent on those physical attributes for it's behavior. Life does not exist independently of the organisms it is the life of, any more that any physical property exists independently of physical entities they are the properties of.
At the physical level, the life of an organism is manifest as a physical process. It is the physical aspects of the process that the sciences of biology study, but it is the attribute of life itself that makes the unique living process possible. Every action of the purely physical (non-living) must be started and stopped in relationship to other physical entities (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 sustains it.
There are five characteristics of the life process that the attribute of life makes possible that cannot be described or explained in terms of physical properties: 1. Self-initiated and self-sustained, 2. Self-determined existence, 3. Sentience, 4. Unity, and 5. Continuity. These are described below:
1. Self-initiated, self-sustained Process
From a physical perspective, only the physical aspects of the life 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 sciences 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.
[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.]
2. Self-determined Existence
Non-living physical entities do not act to sustain themselves as the kind of entities they are. 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). All of a non-living entity's behavior can be explained entirely in terms of physics and chemistry.
The unique character of the life process is to sustain the living organism as the kind of organism it is. So long as that process continues, the organism continues to exist as the kind of organism it is. The moment the process ceases, the organism reverts to being a mere physical entity.
There is no possible physical explanation of why or how a physical entity sustains its own identity except when such an entity is living. Physical attributes alone can neither explain or describe this aspect of the life process.
[NOTE: 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, only because it would otherwise 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 entities.]
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 respond 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, unlike a mere physical reaction which 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 stimuli an organism will respond to and the specific response the organism makes is determined by the organism's nature as an organism, that is, an organism's reactions to stimuli will be appropriate for sustaining the life process and the organism as the kind of organism it 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.)
[NOTE: There is another reason to emphasize the unity of life. The life process is self-sustaining because, as far as can be known, 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" (evidential) basis for this view. If science, as well as philosophy, are to be based on observation, there is no observable evidence that life has ever or can possibly come from non-life.]
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. The reason is the same as that for unity. Since the life process is self sustained, discontinuity would require the life process to cease at which point the organism would cease to be a living organism.
[NOTE: Both unity and continuity are attributes of consciousness as well, and much more important, philosophically, there.]
I have not listed reproduction as a separate attribute of life. It is certainly a unique attribute of organisms which is not possible to non-living entities, but it is only one way that life is self-initiated and self-sustained. Prokaryotes, technically, do not reproduce, they increase in number by a process of cell division, which means they are really genetically immortal. There is also a small jellyfish found in the Mediterranean and in the waters of Japan, turritopsis dohrnii that is genetically immortal. It changes its form to survive changing conditions but returns to its normal genetically identical form and can do this perpetually. Reproduction is the most common way life initiates and sustains itself, but it is not the only way.