As has been discussed previously, the aim of this paper is propose a 'unified theory' of metabolism. If we can define in precise terms how living processes maintain their coherence we shall be better placed to understand how cells, organs and indeed, how brains work. To that end I present the following thought experiment. The thought experiment is not designed to be a precise analysis, rather it is intended to sketch out a preliminary framework that will help to focus our thinking and accentuate potentially relevant themes. We shall attempt a preliminary definition of the life process in terms of its property to persist. At this stage we are not interested in reproduction. Our attention is on the moment by moment process of life within a complex environment.

The Thought Experiment

Consider a strain of bacteria existing in a liquid medium that depends for its survival upon the presence of certain substances in suspension. Competition among the bacteria, and the fact that only a few of the numerous substances that they are capable of metabolising are ever present at any one time, has determined (through the process of natural selection) that they display a certain efficiency in their metabolic chemistry. Simplifying somewhat, the bacteria are equipped with chemical response mechanisms that detect the presence of particular substances in their environment and which then activate the production of substance-specific enzymes in order to metabolise them. The time lag between the detection of a particular substance and the change in the bacteria's chemistry to effect maximum rate of metabolism is, let us say, 10 seconds.

Now suppose that there is a food for this bacterium, substance-A, which occurs in the environment once every 4 or 5 hours, and there is a corresponding occurrence of substance-B (which has no metabolic value to the bacteria at all). Substance-B always occurs in the environment 5 seconds before substance-A. Imagine that a mutation occurs in one of our bacteria such that instead of responding to the presence of substance-A, it begins to respond to the presence of substance-B. The rest of the chemical response mechanism remains unchanged, such that the detection of substance-B results in the production of enzymes to metabolise substance-A. The mutated bacterium now effectively pre-empts the occurrence of substance-A and reaches its maximum rate of metabolism before other bacteria that do not share the same mutation. This, of course, has positive consequences for the bacteriumÕs survival.

In order to determine what has happened we shall first analyse the relationship between substance-A and substance-B. If substance-B invariably occurs just before the occurrence of substance-A, then one can consider that there is an implicit relationship between them - i.e. a relationship between them which is implicit simply in the temporal relationship of their occurrence. The temporal relationship between substance-A and substance-B could be entirely coincidental. We can imagine a species evolving in such a way so as to exploit an environmental condition that turned out to be a long-term anomaly. The temporal relationship between substance-A and substance-B, whether due to some physical relationship actually existing between them or as a consequence of random coincidence, is to be thought of as an example of an 'implicit order' within the context of this analysis. Also, 'implicit' is intended to refer to the fact that the relationship between the substances is discontinuous, that is to say that the temporal relationship between the substances is not to be assumed as necessarily physically or causally related.

The bacterium has increased its potential to survive and has done so by making an explicit physical (or chemical) connection between substance-A and substance-B via its metabolism. At this stage we shall not attempt to define rigorously the term explicit. However, it is to be assumed that, prior to the mutation of the bacterium, the relationship between the substances is discontinuous and is implicit only in as far as there is a temporal relationship between them. Subsequent to the mutation, however, this temporal relationship has become 'realised' or 'explicit' in a more direct and physical way via the metabolism of the bacterium.

To clarify the thought experiment, we need to consider some of its aspects that may appear problematic. First, the particular chemical chain of events within the mutated bacterium that link substance-A and substance-B may be dormant most of the time. It is possible that both substance-A and substance-B may disappear from the environment altogether, never to appear again. But, this would not alter the status of the bacterium as 'living.' So we can not think of the potential for the bacterium to establish, explicitly, through its metabolism a relationship between the substance-A and substance-B as being necessary to its status as 'living.' However, the fact that there are many chemical processes and they represent an, albeit changing, but unbroken continuum of cellular metabolism is necessary to the persistence of the bacterium as a living organism. So, whatever defines the bacterium as 'living' is not to be found in the collection of potential chemical states that may or may not be expressed in the course of its existence, but in the moment-by-moment processes which are the expression of the evolutionary steps that were their cause.

Following from the premise that the persistence of any living process is an example of a single universal principle, we can assume that all the previous actively selected mutations that comprised the evolutionary history of our hypothetical bacterium were examples of the same principle and, therefore, avoid the logical circularity of using a substance already known to be beneficial to the metabolism of the bacterium (substance-A).

Definition of life. Following on from this thought experiment, I suggest a preliminary and tentative 'ontological' definition of the 'Life Process:'

The persistence of the 'Life Process,' which may be chemical, behavioural, neural, perceptual etc., may be understood as resulting from the making explicit of implicit orders, the sum of which (expressed and potential in the case of complex organisms) comprises the 'environmental survival space.'

The thought experiment illustrates a simple relationship between an aspect of the environment (an implicit order) and an aspect or 'mode' of a living organism. The assumption is that this relationship is representative of a hypothetical class of such relationships which may be defined in the same way and which collectively may encompass life processes in all their actual and possible modes. The progress of this argument is a search for the meaning of the term 'explicit' within this context.

The term 'environmental survival space' is to be understood as the entire set of 'implicit' orders within the environment that an organism can potentially 'make explicit' as part of the life process.

	Detection of Substance A by the chemical response mechanism leads to a change in the chemistry of the bacterium such that it can metabolize substance A - Time from detection of Substance A to maximum rate of metabolism is, let us say 10 seconds.
	Mutated bacterium's response mechanism now responds to the presence of Substance B, but the mutation still results in a chemical change such that the bacterium is able to metabolize substance A.

Substance A and Substance B are discontinuous. However, they are implicitly related through the temporal order of their occurrence.

Via the metabolism of the mutated bacterium the temporal relationship between Substance A and Substance B is made explicit - substance A and Substance B become part of a continuous process - the metabolism of the mutated bacterium.