Life has many different meanings, as outlined in any dictionary. It is not possible to find one universal definition covering all meanings. The best one can hope for is to find a common denominator for all these different meanings. In my view, what biologists call "life" is simply communication activity exerted by a communicating compartment (system).

This "simple" answer can be deduced logically as follows: Why does the statement "We are glad because there is born" not make sense? Because we have to say what or who is born. It follows that the terms "life" and "death" make sense only if they are linked to what I call a compartment - what physicists would call a system. Thus, life is a property of some sort of compartment(s) "[1].

What types of compartments occur in nature? That is, what can be alive or can die? Compartments occur in many more forms than one usually realizes. There are compartments restricted to one and the same individual (cell organelles, cells, syncytia, epithelia, etc). There are compartments made up of more than one individual but belonging to the same species, e.g., a heterosexual couple, a population, etc. There are also compartments consisting of individuals of different species, e.g., the members of a host-parasite relationship; a food chain; or, at the highest level, the planetary level, "all life on Earth." Insight into this hierarchical scheme of compartmental organization is essential to understanding the definitions of death and life [2].

When does a given compartment die? How does one define death? A compartment dies at the moment it irreversibly loses its ability to communicate at its highest level of compartmental organization. Only the highest level matters, not the lower ones. This solves the problem of the duality of death: a decapitated chicken is dead even if all of its cells and tissues (its lower levels) are brought into tissue culture and stay alive. It follows that a given compartment starts to live from the moment it acquires the ability to communicate at its highest level of compartmental organization.

Thus, the major difference between a living and a nonliving system is its communication activity. Living systems communicate; nonliving ones do not. Reproduction and evolution are not essential to life as such in the short run, only in the long run.

In order to understand what life is, one has to understand in sufficient detail what communication is and what its purpose is. Briefly,

• Communication is transfer of information.
• This transfer requires a communication system. Such a system typically consists of a sender-encoder, which emits a coded message, a transmission channel (air, blood, etc.), through which the message is transported along a gradient; and a receiver-decoder-amplifier-responder, in which energy must be stockpiled in order to allow an energy-consuming response. Very often, there is feedback.
• What constitutes "information"? A message of whatever nature contains information when, upon being decoded by the receiver, it can cause the mobilization of part of the stockpiled energy to do some sort of work sooner or later.
• What is the purpose of communication? To make the receiver work for the sender.
• Is information material or immaterial? Information in itself is immaterial, but it can need material support in order to be transported. This introduces an immaterial aspect to the definition of life.
• Why are living systems more than the sum of their parts? Because the different levels of compartmental organization are interconnected by specific means of communication, means that are lost when the system is disassembled.
• What is the difference between communication and interaction? Communication requires the decoding of a coded message. In interaction, no coded message is involved.

The definition of life (as an activity), taking into account the fact that life is invariably a trait of some sort of compartment, can be stated as follows:

Life of compartment S at moment t is the sum total of all acts of communication (C) performed by this compartment, from its lowest level of compartmental organization (1) to its highest one (j), at moment t. An act of communication is the cascade of events, from the release of a coded message by a sender, over its transport through a communication channel, to the energy-consuming response in the receiver. In symbolic notation:

Using this approach, it is possible to define artificial life and to answer the question as to whether a computer, a virus, or a prion are alive or not. A well-functioning computer (a mechanical extension of our brain) is alive; a virus is not.

The difference between "normal" carbon-chemistry-based life versus "artificial" (silicon- and metal-based) life can be shown as follows:

where STC is the type of chemistry used by compartment S and TE the type of electricity (carried by electrons or by ions).

According to this approach, life is not a state and not a machine, but the activity of a machine. This definition automatically implies that life uses both hardware and software. The continuation of life requires a double continuum: that of its hardware and that of the software (needed to decode the incoming messages).

This concept allows the introduction of ethical positions into biology because it makes it possible to define the very moment that human (and other life) begins and ends. This ability is important for discussions about abortion, euthanasia, organ donation, and adoption. It also allows us to develop a theory of evolution more complete than neo-Darwinism, which does not provide a biological basis for teaching and learning or for free will. What is called "cultural evolution" is largely evolution by teaching and learning (mainly software evolution). Furthermore, Darwinism overlooks an important aspect of self-selection. According to the double-continuum theory, selection is a result (of gradient formation), not a cause.

A good way to demonstrate the concept of life as communication to an undergraduate (or even high-school) class is to use, as an example, a musical instrument:

I take a police whistle and put it on the desk. Question: Why does it not produce a melody? Answer: You have to blow into it. I take the whistle, close the outlet hole with my finger, and blow. Nothing happens. Why not? You have to open the outlet hole! I do that and produce a sound. Thus, one needs an instrument, but the instrument has to have functional holes. Why do I have to blow? By blowing I compress air (gas), which, upon expansion, will liberate energy. This is a means of storing energy in the whistle, which acts as a sender. Thus, communication requires pre-stored energy as well. I further explain that the air medium between the whistle and the ears of the students is called the "transmission channel." Next I explain what is needed in the receivers (the students): receptors, a message decoder, and a response. This analysis allows one to deduce the definition of information.

Next arises an important question about variability: how many melodies can one produce with such a simple instrument? Many: loud, soft, long, short, etc. I have to decide how I will play - communication involves decision making.

The biological counterpart of the instrument is the cell. The metal of the whistle is the equivalent of the plasma membrane. The functional holes are the ion channels, etc. The action of the ion pumps is the equivalent of blowing air into the whistle. The energy stored in the compressed air is the equivalent of the energy stored in ionic gradients (electricity) in the cell. In my opinion, Nature's greatest invention was to make electricity out of water and a simple salt solution and to use that electricity for communication. Life is to a large extent an electrical phenomenon.

This concept allows us to teach biology using a much more logical method than is found in most textbooks. Our students love this approach.

This view of life has profound consequences for the theory of evolution. Neo-Darwinism explains only the evolution of the instruments of communication - the hardware. At least as important is the evolution of the software. But this type of evolution is not governed by the laws of genetics, but by those of communication. By logical deduction, the conclusion can be reached that the driving force of evolution is not selection, but communication activity (that is, life itself). Selection is not a cause, but is itself a result of something that happened before. When I made this deduction a few years ago, I could not believe it myself - but there is no way around it.