The Parable of the Ichthropic Principle
Sixty meters underground, a river used to run through the limestone of the Mexican state of Oaxaca. Because the limestone was uneven in density and porosity, the river carved an irregular channel, widening and contracting. Eventually, over a very long period of time, the surface of the land above underwent changes resulting from diminished rainfall. As the volume of water draining through the underground river decreased, the channel it had carved became a cave. Nonetheless, a trickle of rain still flowed through cracks and crevices, enough to maintain stable pools of fresh water in the lightless depths.
In one such pool lived a small school of fish of the family Characidae. Characids are an adaptable group, occupying many ecological niches of the planet Earth. The characids of this geologically isolated pool had several distinctive adaptations, the most unusual being that they were eyeless. Thus we can identify them henceforth as blind cave fish. Lacking sight, the blind cave fish were well equipped to detect vibrations of any sort through the sensory cells of their lateral lines, which was how they foraged for food as well as how they located each other for mating purposes. Fortunately, water is a superb transmitter of vibrations. Greater self-awareness would not have been adaptive in the bleak conditions of their pool, but if they'd possessed it, they would have had no reason to suppose that any other characids inhabited any other pool in this or any other cave, or indeed that any other pool in any other cave was inhabitable.
The blind cave fish had two rigid requirements for survival--oxygen and food. The oxygen in the pool was maintained at roughly the level they required by the dependable trickle of rain which replenished the loss of water through the porous limestone bed of the pool. Also, the water was cold--a constant forty-one degrees Fahrenheit--which of course allowed maximal oxygenation. Although the fish had no "knowledge" of it, a grave danger to their survival existed in two kinds of pollution: nitrates from their own metabolic waste products, and gradual mineralization from the influx of acid rain water seeping through the soil. Periodically, however, drenching storms would flood the land, refilling the underground river channel and flushing the pool. Most of the blind cave fish would be swept away to an uncertain fate, but enough would survive to rebuild their population. Even the most catastrophic flushing would not decimate them, since their eggs, which were adhesive, were always laid in protected chinks and cracks. Had the floor of the pool been smooth, or had the flooding carried other menaces into their cave, no doubt the blind cave fish couldn't have thrived as they did. It should be noted that the thick layers of rock above, which shielded them from hot sunlight, also shielded them from ultra-violet and other forms of radiation that might have threatened their survival.
The blind cave fish were dependent for their nutrition on another intricate and improbable set of circumstances. Since no light whatsoever penetrated the cavern, no photosynthesizing plants or algae could flourish. Nonetheless, populations of microbes and nearly microscopic arthropods shared the pool. These were the food source upon which the blind cave fish depended, though they supplemented their diet by scavenging the corpses of their own dead. In turn the arthropods and microbes were dependent on bat droppings for 100% of their nutrients. The bats, in huge numbers, infested a large dry cavern of the same cave. The only above-water outlet from their cavern to the fresh air above passed through the grotto of the pool, the ceiling of which was too encrusted with stalactites to attract bats to nest. Thus the quantity of guano the bats dropped in flight was always enough to sustain the pool's organisms yet never enough to poison the water.
The blind cave fish were by far the largest and most metabolically active of these aquatic creatures. Having neither predators nor competition, they had ample reason to be happy with their lives--that is, had they had enough self-awareness to exhibit happiness--since each and every condition of their environment seemed specifically suitable to their needs, while any variation of those conditions would have made their life impossible. Indeed, the conditions in which they subsisted were so random yet so improbably assembled that it must have seemed to the fish--again granting them the self-awareness to consider probabilities--that the pool had been designed to provide for their existence.
Allowing them just a bit more rationality than they truly possessed, logic would surely have suggested to the blind cave fish that where there is design, there must be a designer. No matter how much intellect we attribute to our three-inch long albino eyeless characids, however, it's clear they had no means of fathoming the nature of the designer, unless it were itself an inscrutable but omnipotent blind cave fish.