Striving for the perfect balance between Eros and Thanatos is the very stuff of life, says Eric Harrison.

To be alive seems to be very different from being dead. Yet, when we look more closely at our bodies, the situation is not so clear cut at all. Millions of cells within us die every day, and millions are born. Even more intriguing is that most of these cells are not "us", not even the mitochondria that are our internal powerhouses. These millions of tiny animals and plants have entirely different genetic material from us, and yet we wouldn't last a single minute without them.

Furthermore, we rely on these multitudes of us and them to die at exactly the right time, with no delays. There are no old folks homes within our bodies to care for unproductive, geriatric cells to gently fade away. For the greater good, we demand that they die and get recycled as soon as they put down their tools. To be alive involves a constant interplay of opposing biological forces whose work is to maintain balance, or "homeostasis". For example, adrenalin stimulates us into action each morning until signals tell us that enough is enough. Then negative feedback, which we tend to notice as fatigue, checks that rising adrenalin, and pulls us back towards balance. These on-off processes within our bodies are continually calibrating levels of blood sugar, acidity, muscle tension, temperature, and so on.

Similarly, with life and death in the body. Growth is not "good" and decay "bad". They keep each other in check. The birth of millions of new cells daily through cell division is obviously a good thing. Otherwise, we would soon wither away. Yet, unbridled growth is a horror. We call it cancer. These are the cells that know the secret of eternal youth. They can remain juveniles forever, and continue to breed like rabbits. Well behaved cells, on the other hand, know their age and die off as soon as they are no longer needed. They press their own internal suicide button for the good of the larger organism, or are persuaded to do so by other forces in the body.

This process is called "apoptosis" or "programmed cell death". The Latin word literally means "falling away", the way a leaf drops from a tree. In other words, for me to be alive relies on millions of bits of me dying (and other bits being born) each day. We need death to live at all.

Centuries ago, this truth was illustrated by paintings and drawings of "Death and the Maiden". These depicted a leering skeleton embracing and being embraced by a buxom wench. These are not the eternal enemies we assume them to be. At a gut level, they actually desire and need each other. Similarly, Freud described life as a continual psychological struggle between Eros and Thanatos, between the lifeforce and the impulse towards death, between the urge to activity and the urge to rest.

Sometimes Eros is high. We want to take on the world, particularly when we are young. At other times, we just want to curl up and watch TV. Furthermore, neither impulse is inherently good or bad, although both are disastrous in excess.

Eros is good, but too much of it is as dangerous as a methamphetamine high. We also need a comfortable familiarity with Thanatos, with rest and inactivity, with letting go and letting things "fall away", to have a good life.

We can easily see Thanatos and Eros, Death and the Maiden, playing out their dramas within us. Eros is fuelled by dopamine, the "anticipation and reward" hormone. This gives us the kind of enthusiasm and focus that stimulates us into action and is essential for any kind of achievement.

While it is obviously good to strive for our goals, too much dopamine can make us restless and confused, chasing a glorious future that never arrives. The antidote to this hyperactivity is beta-endorphin, the "sit back and enjoy what you've got" hormone that is the agent of Thanatos.

To "live for the present and take each day as it comes" has much to recommend it. Just like dopamine, though, beta-endorphin in excess can also have its down sides: lethargy, depression, and a poor, sick old age.

In reality, we are rarely well balanced between Eros and Thanatos. We tend to have an excess of one or the other, usually the latter. One problem with being too alive is that it often hurts. Little children, bursting with Eros, are acutely exposed to pain and disappointment: "After laughter comes tears."

Children are frequently howling because they feel both the good and the bad too intensely. After the ecstasy, the crash. Yet after a few minutes, Eros will get them on their feet again, as enthusiastic as ever, racing joyfully towards the next crash.

As adults, we typically become more calculating and fearful. Rather than risk disappointment, we stop trying. We choose to be less alive rather than face being hurt yet again. At any age, we become cynical and world weary. Ennui sets in. We make jokes about chocolate and beer being better than sex and relationships.

People become less than fully alive in many different ways. Those who have been severely traumatised can become emotionally numb forever. The rest of us can easily dull our feelings with food or drugs, mindless entertainment or the pursuit of money and possessions. So long as we are cheerful, friendly and harmless, no one minds if we are devoid of any real feeling. And if other people seem to like us, why should we care?

Yet when this strategy no longer satisfies, we hear people complain, "I feel only half alive." Or "You call this a life? This is not living. It's just existing!" To make matters worse, we can all remember days that were more bright and beautiful than today. We were all young once. At least the music was so much better then.

If we feel only half alive, then coffee and other stimulants will certainly give us a temporary zing. Advertisers will try to persuade us that the latest herb from the Andes, or even an electric duster, will make us insanely happy. Even a completely unfounded hope will stimulate dopamine, the "anticipation and reward" hormone, and lift our spirits for a while.

Of course, the dopamine effect rapidly wears off and can't be relied on. Researchers into happiness usually say that our baseline capacity to enjoy life or not is relatively fixed. To suddenly win the lottery, or lose a leg, doesn't make us that much happier or sadder. We soon adjust to our familiar level. But this is only half the truth.

How alive (or dead) we feel is remarkably stable regardless of circumstances, but it is still fluid. It is gradually changing one way or the other at a glacial pace. Our habitual level has been established by thousands of automatic responses to situations in the past. We maintain that level by repeating our responses from last time. This gives us a sense of comfort, safety and "rightness", but it does have a price.

If we habitually respond with new events with something like that open hearted, unguarded sensitivity that we see in little children, we can continue to feel alive right into old age. Our natural relationship to the world will be one of Eros, or love. Conversely, if we habitually respond with caution and reserve, we gradually lose our capacity to feel much at all. Our responses will be governed by Thanatos, or fear.

Fortunately, if our habits are not serving us well, they can be changed, albeit with difficulty. The first step is become more conscious of our automatic reactions in particular situations. If we can "see" how we react, and how that makes us feel, we can choose to be more alive and open if we wish. We can choose to feel more, even if it hurts. We can't completely overrule the conditioning in our body and mind, but we can always make yet another little adjustment for the better.

Unfortunately, we can't just rewrite our thoughts. Habits are embedded in the body long before we are self aware enough to know what is happening. If we habitually respond to the world with worry and suspicion, we are likely to frown, clench our teeth, hold our breaths, hunch our shoulders and knot up in the stomach so often that our posture reflects our character. A frozen, locked up body will guarantee a fearful response to the world, regardless of our thoughts. Fortunately, all these effects can be ameliorated. It is not hard to smile, soften our faces, loosen up our breathing, drop our shoulders, and so on. There is nothing complicated about this, but we need to do it thousands and thousands of times until the old pattern gives way to the new. Changing habits is a physical training, like learning an instrument or playing a sport. It is quite possible to feel more alive. We just have to know what to do and keep at it.