This essay is presented from the perspective of the philosophy of probability.
It is of the nature of a horse to have two eyes. Such is the ancient and naive ‘explanation’. However, that is not an explanation or even an answer. At best, it is simply a statement of the observation, which prompts the question. At worst, it is a copout, which evades the question.
More recent observations are that predatory animals have two eyes in a plane enabling binary vision. In contrast, animals, like the horse, which are prey to others, have eye placement approximately in two planes, which are the two sides of their heads. Such placement affords nearly a complete view of the sphere from which attack by a predator may occur.
In prey, the placement of the eyes approximately forms the axis of a globe of visual sensation, over which the eyes are uniformly distributed. In the case of fewest eyes, namely two, the eyes are at the axis of the globe. The axis may be viewed as the diameter of one longitudinal circumference.
The more eyes distributed uniformly over the globe of visual sensation, the more complete and uniform would be the monitoring of the sphere of predatory attack. Let the algorithm of adding uniformly distributed eyes to the globe of visual sensation be by adding equally spaced longitudinal circumferences, where each circumference has a number of eyes equaling twice the number of circumferences, while maintaining the location of the two original eyes at the axis of the globe.
By this algorithm, the relationship between the number of equally spaced eyes, N, and the number of longitudinal circumferences, n, would be: N = (n^2 – (n -1)) × 2. For n = 1, 2, 3 and 4, the number of uniformly distributed eyes, N, is 2, 6, 14 and 26. The practical upper limit would be determined by the size of the eyes and the size of a horse’s head. Also, the practical number of eyes would be diminished by those of the virtual globe of visual sensation which would be eliminated due to the neck of the horse.
Why do horses have two eyes? Horses have two eyes as a uniform distribution of visual sensors forming a global base for monitoring an attack from any point of the sphere of predation.
It should be apparent that the fact that animals of prey in the scope of human observation, such as the horse, have just two eyes is one possibility among several. It happens that in our universe the number is two. However, the fact that our observation is limited to our earth in our universe voids the rationale, which claims that it is of some fundamental character of a horse that it has two eyes. Indeed, it has two eyes in our universe, but horses may have any range of a number of eyes, notably greater than two, in other regions of the multiverse. It is the multiverse which explains the fact that we observe within our universe just one of many possibilities, where in accord with probability, the number of eyes of earth horses is two.
It is evident that the existence of the multiverse in cosmology is not a consequence solely of the science of physics and the numerical values of the physical constants in our universe. The multiverse is harmonious with biology as well and with what seems like a simple question, ‘Why do horses have two eyes?’