[Back to Human Identity]

Introduction

People argue about the precise definition of life, and many different definitions have been produced. But we don’t need a formal definition: for the purpose of this discussion, we simply need a description, so we can know how to recognise it, and so we can talk in an informed and constructive way about it. And we don’t need to describe life, an abstraction; it is sufficient to describe a living organism, the most basic unit of life. Here is one possible starting point.

• A boundaried physical system which can reproduce in the right environment can be described as a living organism.

As I say, this is a description, not a definition. Other things can also be described as living organisms, but they are all intimately connected with organisms which fit this description. Most organisms cannot reproduce most of the time: this is often because they are too young or too old or too damaged, but you get the occasional oddity like the mule – a sterile cross between a horse and a donkey.

This description naturally leads us to two fundamental drives which direct the activity of all living organisms; these can be described as success and survival.

It is a physical system. Let’s not get confused at this stage with questions about the possibility of virtual life or the creation of a sentient AI: every living organism we have encountered to date has been physical in nature.

It is a boundaried physical system. Without a recognisable boundary, we can neither recognise an organism, nor tell when it has reproduced.

It can reproduce. Reproduction is the most fundamental characteristic of life; all the other commonly identified characteristics of life only matter because they are needed in order for the organism to reproduce.  We may not be able to define life, but for a living organism, success is basically defined as effective reproduction - that is, the creation of offspring which can themselves reproduce.

It must survive.  The organism must survive for as long as is required in order to successfully reproduce: this differs from one organism to another.  Some organisms (like salmon and octupi) die after reproducing, while other organisms (like humans) need to care for their children - or even their grandchildren - to ensure their success.  Survival involves two fundamental (but quite different) activities: feeding, and avoiding harm.

From this perspective, success and survival can be seen as the two basic drives shared by all living organisms.

Attributes

Every living organism engages in certain activities, and has certain attributes: it reproduces, it grows, it feeds, it has feelings, it has desires, and it has purpose. How does a machine, whether it is a car, a computer or an AI, compare?

It reproduces. Even at its most simple, this is an incredibly complicated process.  An amoeba must grow large enough, then copy every strand of DNA once - not two or three times - then gather one copy of each strand in one place, and the other copy in another place, then divide the nucleus so that each of these places is in one half, then divide the cytoplasm so that each nucleus is in one half.  Sexual reproduction is far more complicated.  The cells in complex sexual organisms are constantly reproducing, even when the organism is 'simply' surviving.  Machines do not reproduce.

It grows. Anything which successfully reproduces must also grow; cells grow and divide and grow again. In complex organisms, cells must not only divide, but also differentiate, when appropriate, to create many diffeent kinds of tissue and organ.  Machines do not grow.

It feeds.  Food is not just a source of energy: the organism takes part of the environment, and transforms this food into itself.   And every living organism has specific nutritional needs: a cow can eat grass, a lion cannot; a Koala will eat Eucalyptus leaves, but not Sycamore leaves.  Feeding and processing the food are very significant activities for any living organism: much of its activity consists of feeding and processing the food, then using the nutrients obtained from the food to grow and replace cells.  No mechanical or virtual device engages in anything at all comparable: the petrol you put into your car does not turn into the tyres or crankshaft; a computer can be said to 'feed' upon the electricity supplied to it, but this electricity does not become transformed into the electronic components it is made of. 

It has identity.  Every living organism must distinguish between me and not-me; feeding is fundamentally the process of turning things that are not me into me.  And all the differnt parts of me are also me: the many and varied auto-immune diseases are simply the result of failing to distinguish correctly between me and not-me.  A surgeon can replace your kidney, but the replacement is always someone else's kidney, with their DNA in every cell.  In contrast, a machine (and every part of a machine) has no identity: it simply does what it was built to do; if it stops working, we simply replace it.

It has parts.  The simplest living cells are incredibly complex, with many different parts (including DNA and organelles) all acting and interacting in ways we are only just beginning to understand.  More complex organisms have many organs, which again interact in complicated ways; these organs are made up of many cells, all of which need both to stay alive and to contribute to the organ and the organism as required.  Machines have parts, but those parts are replacable: you change the tyres on your car, the memory on your computer, the only thing which matters is: does the part work?

It has connections.  While the organism has a physical boundary, that boundary is permeable. Most obviously, food is taken in and becomes part of the organism.  The organism is connected to its ecosystem in many ways, not only through what it feeds on and what it excretes; the ecosystem is connected to the planet, the planet is connected to the solar system. That strawberry you ate gained nutrients from the soil, carbon from the air, and energy from the sun.  Machines have a hard physical boundary; while their environment can affect them, they have no ecosystem, they are not a part of their environment in the same way.

It has feelings.  The organism feels, or 'perceives', its environment - that is, the environment has an impact, changing it in ways it can detect; the organism also feels its internal state, mostly in ways we are not consciously aware of (such as the level of our blood sugar), and these are not entirely distinct activities.   Some feelings are neutral, but many feelings are good (indicate a desired state) or bad (indicate a state to avoid).  Two fundamental feelings are fear and hunger, telling the organism that it needs to feed or avoid hard - avoid becoming food.  Of course, in primitive organisms, this will not be fear and hunger as experienced by humans, but the equivalent for the organism in question.  A living organism feeling the temperature or tasting the food is quite different from a machine receiving a data feed containing a number which represents the temperature, or a chemical analysis indicating the constituents of the food.

It has desires.  The organism desires success, and it desires to avoid fear and hunger.  These desires are inherent, not imposed; they are built into the fabric of its being.  A machine has no inherent desires: it cannot reproduce, and it cannot experience fear or hunger; it has has no reason to avoid dangerous situations unless instructed to do so.

It has purpose.  Every living organism has a basic built-in purpose: success; and success requires survival.  It aims to reproduce, and it needs to survive until it has reproduced.  And every living organism acts with purpose, which is not only about success and survival directly, but also indirectly, through satisfying its desires.  Again, these are all innate, buit-in to every cell of every living organism.  In comparison, a machine, computer or AI can only do what it is instructed to do: it may be said to have a purpose, but that purpose is simply what some organism created it for; the purpose is given to it, imposed from outside - the purpose does not arise from within.

More About Fear and Hunger

The most primitive life was a single cell and it fed, of course, on inorganic matter. Escaping harm and finding food were largely a matter of luck, but even a small ability to respond to the environment would convey an advantage if it enabled the organism to avoid harm (by coping with a less than ideal environment or moving to a better one) or find and absorb food (by locating it more effectively, or by making use of a wider range of food).

Of course, the organism cannot respond to the environment without first perceiving it, and living organisms have developed many techniques for perceiving their environment. If there is only one mode of perception (or ‘sense’, perceiving properties such as salinity, brightness or temperature), the response can be simplistic – perhaps move towards brightness, or away from heat.

But even if there is only one sense which is used to perceive the external environment, the organism must have some perception of what is happening internally – we generally call this ‘interoception’.  Even single-celled organisms must possess interoception: they must perceive their internal state sufficiently well in order to determine whether now is the right time to initiate binary fission.  Of course, ‘perceive’ in this context does not imply anything we would recognise as mental activity.

So even the simplest living organism must have multiple senses, but the organism still needs to respond, to act in a way which is appropriate, given all those perceptions, so there must be some means of integrating the differing perceptions into a single ‘something’ which the organism can respond to.  In higher forms of life, we might describe this ‘something’ as the model the organism has of itself and its environment; while it must exist, there seems to be no equivalent term for this in the lower forms of life.

Also, in higher forms of life, we can talk about the ‘executive function’ which enables the organism to take this integrated model and determine what action is most likely to enable it to avoid harm and absorb food.  But, again, it is clear - whatever words we choose to use - that even the simplest living organisms must have the ability to perform this executive function to use its model and decide how to act.

Later organisms could develop the ability to feed on organic material, living or dead. And later still, multicellular organisms developed. But, however sophisticated life became, the two basic concerns, avoiding harm and finding food, always remained essential.

Responding to Fear and Hunger

These two concerns are very different; while they are both always important, they operate very differently.  Avoiding harm is a constant: if something – anything – consumes you, crushes you or cooks you, then it is game over.  Once dead, you can’t reproduce.  But if you have fed well, you may not need to eat again for a while; and if you fail to find food here, you simply continue to look – perhaps it will be around the next corner; only a repeated failure to find food is fatal.

So you must seek to avoid harm at all times, but you only need to find food sufficiently frequently: it doesn’t matter that much if you go hungry sometimes, or even if you go hungry often, as long as you succeed in reproducing.

In order to avoid harm, you need an integrated model of its internal state and external enviroment.  It is helpful for your model to include some way to assess the level of danger you face: sometimes the risk is low, and the executive function can focus on other priorities; at other times, the risk is high, and then it is important to take action to avoid harm. Such action may only be required occasionally, but you must constantly monitor whether the action is required.

These two basic concerns have different, and sometimes conflicting priorities. If you are in danger, then you need to prioritise safety above finding food; on the other hand, if you are sufficiently hungry, then you will be willing to risk your safety to find food. So, as well as recognising and responding to these concerns, you must assess their severity and then balance the risks in order to form a judgement about your best course of action: this is all part of the executive function built into all living organisms.

In more advanced creatures, when we recognise the need to avoid harm, we call this ‘fear’; when we recognise the need to feed, we call this ‘hunger’. There is a great deal going on when we experience fear and hunger; but, whatever else is happening, we can find in these experiences a basic continuity with the fundamental realities of life experienced by the earliest living organisms.

Full Life

All of this discussion is about the things which characterize real life, as opposed to artificial or virtual life.  It does not attempt to describe everything which living organisms are capable of.  At one point, Jesus talks about 'life in all its fullness', which we can reasonably assume encompasses much more than just feeding well, finding pleasure and avoiding pain.  In fact, His teaching and example shows that it is possible to transcend these concerns - perhaps not completely, and not all the time, but they do not have to control our lives.  However, such considerations are not part of this article.

More Precisely

A boundaried physical system which is capable of reproducing in the right environment is a living organism. Other similar boundaried physical systems are also living organisms, despite being sterile. There are four basic reasons why a living organism may be sterile:

• it may be damaged in some way;
• it may suffer from genetic errors;
• it may have been produced by an attempt at reproduction between living organisms which cannot produce fertile living organisms; or
• it may help other living organisms to reproduce.

Some parts and products of living organisms can be alive without being a living organism.  For example, a gamete (such as a human egg or sperm) can be alive, but is not itself a living organism; an organ which is to be transplanted into another body can also be alive, but is not a living organism.  Viruses are also products of living organisms, albeit products which have hijacked some part of the organism in question.  In all these cases, it is simple to tell the difference between a living example and a dead one.

So, within this framework, it is reasonable to describe a viable virus as being alive. But people do like to argue about whether viruses are alive.  It seems to me that the question is ultimately unimportant: you can define 'alive' in whatever way is most convenient for the context in which you are working.  They are not living organisms (as described above); on the other hand, they clearly belong to the 'living matter' category - they can be killed, and in a universe without life, viruses could not exist.

We say that success and survival are the two basic drives of all living organisms; and we talk of reproduction rather than sex: sex is only the mechanism of reproduction for complex living organisms, while the simpler ones are content to reproduce by cell division.  And we have already noted that there some living organisms are sterile.  So a more accurate, but less alliterative, statement would be, "Reproduction and survival are the two basic activities of all non-sterile living organisms."

One final technicality: it is clearly the case that some people choose not to reproduce, and some people are not allowed to reproduce.  The point is that these people (these 'living organisms') all have these two fundamental drives - success and survival.  We can choose to die, and we can choose to be childless, but the fact that we can over-ride these drives does not mean they are missing or unimportant.

Notes

The awkward language is deliberate: it is simpler (but, I suggest, less helpful) to offer a definition of life.  Many people have done just that.

Sara Imari Walker (like NASA) defines life as "a self-sustaining chemical system capable of Darwinian evolution", but this is problematic in several ways - please see the New Scientist article, "How a radical redefinition of life could help us find aliens".  Apart from any other consideration, it should be obvious that no life is self-sustaining.  But at least they recognize that it is a chemical system, implicitly ruling out virtual creations.

George Musser in "The Biologist Blowing Our Minds" about Michael Levin "uncovering the incredible, latent abilities of living things" is worth a read; possibly the most relevant part addresses the question, "Why don't robots get cancer?"  A key observation - "The robot may or may not be intelligent to some degree, but at the next level down, all the parts are passive; they don’t have any goals of their own."

Jensen Suther's article in the New Statesman, 'Hegel against the machines', describes several earlier expressions of the view expressed here that intelligence is an attribute of life, so machines (non-living mechanisms, whether mechanical or electronic) cannot think or be intelligent.  Dreyfus ('What Computers Can’t Do', 1972 and 'What Computers Still Can’t Do', 1992) and Haugeland ('Artificial Intelligence: The Very Idea', 1985)  are reasonably well known, but Suther points out that Hegel identified the issues even more clearly in works such as 'The Science of Logic' back in 1812.

(Version 4, 18 July 2025)