The shifting configurations of genetic material exist in an unpredictable dynamic system much like the weather. But, like the weather, the range of permutations is not infinite: it is contained within a distinctly bounded range of possibilities. However impossible it may be to predict the precise atmospheric conditions in a month's time, it will never be the case that it will rain scissors and false teeth. Similarly, a breeding enclave of field mice will not give rise to a cluster of amethysts. Within these boundaries, however, an astonishing variety can manifest itself, and any particular outcome within this variety has no possibility of being anticipated. How exotic and incomprehensible all these fleshy, awkward humans would be to the elegant array of trilobites that populated the deep and distant past.

Similarly, memory is a rich, dense compost heap, encompassing so much more than its superficial layers can reveal. The introduction of the subtlest sensory input can instigate a long-buried experience to emerge distinctly into primary consciousness once again. Things that were inaccessible and ignored for decades will suddenly re-appear with utter clarity. Where were they hidden? Why do these interred impressions persist for so long? What else is concealed within the gloom? Is every minute fragment of sensory input stored somewhere, remaining dormant until the appropriate catalyst coaxes it out of hiding? Why are some completely frivolous details retained for a lifetime, while distinct events are subsumed within a few weeks? Our subjective evaluation of importance is clearly not the guiding principle at work. How much of our existence is about us anyway? This vast, teeming society of builders and regulators and maintenance crews has no association at all with our values or ambitions. We are nothing but oblivious passengers carried along by forces too subtle and too profound for us to recognise, much less comprehend or control. Our dreams give us a glimpse of this: so many people, places and events that exist fully outside of our experience are conjured up for us night after night. The most a passenger can do is observe and try to enjoy the journey.