설계 논증(argument from design)에 대해서는 아래 글들을 참조하라.
검증 방법: 1. 설계의 논증 --- 진화 심리학 첫걸음마
설계 논증의 역사는 아주 오래 전으로 거슬러 올라가지만 페일리(William Paley)의 『Natural Theology(자연 신학)』가 제일 유명한 것 같다.
페일리의 논증을 요약하면 이렇다. 시계 같이 어떤 기능(시각을 알리는 것)을 잘 행하도록 정교한 구조로 만들어진 것이 “저절로” 생겼다고 보기는 힘들다. 누군가 지적인 존재가 만들었다고 보아야 한다. 동물의 눈은 시계보다 훨씬 더 정교하며 어떤 기능(보는 것)을 잘 행하도록 생겨먹었다. 따라서 눈이 “저절로” 생겼다고 보기는 힘들다. 누군가 지적인 존재가 만들었다고 보아야 한다. 인간보다 우월한 존재 즉 신이 만들었다고 보는 것이 가장 합당하다. 이로써 신의 존재가 입증된다.
페일리의 글을 직접 읽어 보자.
STATE OF THE ARGUMENT
In crossing a heath, suppose I pitched my foot against a stone and were asked how the stone came to be there, I might possibly answer that for anything I knew to the contrary it had lain there forever; nor would it, perhaps, be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place, I should hardly think of the answer which I had before given, that for anything I knew the watch might have always been there. Yet why should not this answer serve for the watch as well as for the stone; why is it not admissible in the second case as it was in the first? For this reason, and for no other, namely, that when we come to inspect the watch, we perceive—what we could not discover in the stone—that its several parts are framed and put together for a purpose, e.g., that they are so formed and adjusted as to produce motion, and that motion so regulated to point out the hour of the day; that if the different parts had been differently shaped from what they are, or placed after any other manner or in any other order than that in which they are placed, either no motion at all would have answered the use that is now served by it. This mechanism being observed—it requires indeed an examination of the instrument, and perhaps some previous knowledge of the subject, to perceive and understand it; but being once, as we have said, observed and understood—the inference we think is inevitable, that the watch must have had a maker—that there must have existed, at some time and at some place or other, an artificer [designer] or artificers who formed it for the purpose which we find it actually to answer, who completely comprehended its construction and designed its use.
APPLICATION OF THE ARGUMENT
Contrivances [designs] of nature surpass the contrivances of art in complexity, subtlety, and curiosity of the mechanism; and still more, if possible, do they go beyond them in number and variety; yet, in a multitude of cases, are not less evidently mechanical, not less evidently contrivances, not less evidently accommodated to their end or suited to their office than are the most perfect productions of human ingenuity.
I know no better method of introducing so large a subject than that of comparing a single thing with a single thing: an eye, for example, with a telescope. As far as the examination of the instrument goes, there is precisely the same proof that the eye was made for vision as there is that the telescope was made for assisting it. They are made upon the same principles, both being adjusted to the laws by which the transmission and refraction of rays of light are regulated. I speak not of the origin of the laws themselves; but such laws being fixed, the construction in both cases is adapted to them. For instance, these laws require, in order to produce the same effect, that rays of light in passing from water into the eye should be refracted by a more convex surface than when it passes out of air into the eye. Accordingly, we find that the eye of a fish, in that part of it called the crystalline lens, is much rounder than the eye of terrestrial animals. What plainer manifestation of design can there be than this difference? What could a mathematical instrument maker have done more to show his knowledge of his principle, his application of that knowledge, his suiting of his means to his end—I will not say to display the compass or excellence of his skill and art, for in these all comparison is indecorous, but to testify counsel, choice, consideration, purpose?
To some it may appear a difference sufficient to destroy all similitude between the eye and the telescope, that the one is a perceiving organ, the other an unperceiving instrument. The fact is they are both instruments.
But, up to the limit, the reasoning is as clear and certain in the one case as in the other. In the example before us it is a matter of certainty, because it is a matter which experience and observation demonstrate, that the formation of an image at the bottom of the eye is necessary to perfect vision. The formation then of such an image being necessary—no matter how—to the sense of sight and the exercise of that sense, the apparatus by which it is formed is constructed and put together not only with infinitely more art, but upon the selfsame principles of art as in the telescope or the camera obscura. The perception arising from the image may be laid out of the question; for the production of the image, these are instruments of the same kind. The end is the same, the means are the same. The purpose in both is alike, the contrivance [design] for accomplishing that purpose is in both alike. The lenses of the telescopes and humors of the eye bear a complete resemblance to one another, in their figure, their position, and in their power over the rays of light, namely in bringing each pencil to a point at the right distance from the lens; namely in the eye, at the exact place where the membrane is spread to receive it. How is it possible, under circumstances of such close affinity, and under the operation of equal evidence, to exclude contrivance [invention] from the one, yet to acknowledge the proof of contrivance having been employed, as the plainest and clearest of all propositions, in the other?
Observe a newborn child first lifting up its eyelids. What does the opening of the curtain discover? The anterior part of two pellucid [transparent] globes, which, when they come to be examined, are found to be constructed upon strict optical principles—the selfsame principles upon which we ourselves construct optical instruments. We find them perfect for the purpose of forming an image by refraction, composed of parts executing different offices, one part having fulfilled its office upon the pencil of light, delivering it over to the action of another part, that to a third, and so onward: the progressive action depending for its success upon the nicest and minutest adjustment of the parts concerned, yet these parts so in fact adjusted as to produce, not by a simple action or effect but by a combination of actions and effects, the result which is ultimately wanted. And forasmuch as this organ would have to operate under different circumstances with strong degrees of light and with weak degrees upon near objects and upon remote ones, and these differences demanded, according to the laws by which the transmission of light is regulated, a corresponding diversity of structure—that the aperture [opening], for example, through which the light passes should be larger or less, the lenses rounder or flatter, or that their distance from the tablet upon which the picture is delineated should be shortened or lengthened—this I say, being the case, and the difficulty to which the eye was to be adapted, we find its several parts capable of being occasionally changed, and a most artificial apparatus provided to produce that change. This is far beyond the common regulator of a watch, which requires the touch of a foreign hand to set it; but it is not altogether unlike Harrison's contrivance for making a watch regulate itself, by inserting within it a machinery which, by the artful use of the different expansion of metals, preserves the equability of the motion under all the various temperatures of heat and cold in which the instrument may happen to be placed. The ingenuity of this last contrivance [invention] has been justly praised. Shall, therefore, a structure which differs from it chiefly by surpassing it be accounted no contrivance at all; or, if it be a contrivance, that it is without a contriver [inventor]?
To the marks of contrivance discoverable in animal bodies, and to the argument deduced from them in proof of design and of a designing Creator, this turn is sometimes attempted to be given, namely, that the parts were not intended for the use, but that the use arose out of the parts. This distinction is intelligible. A cabinetmaker rubs his mahogany with fish skin; yet it would be too much to assert that the skin of the dogfish was made rough and granulated on purpose for the polishing of wood, and the use of cabinetmakers. Therefore the distinction is intelligible. But I think that there is very little place for it in the works of nature. [ ]
All that there is of the appearance of reason in this way of considering the subject is that, in some cases, the organization seems to determine the habits of the animal and its choice to a particular mode of life which in a certain sense may be called "the use arising out of the part." Now, to all the instances in which there is any place for this suggestion, it may be replied that the organization determines the animal to habits beneficial and salutary to itself, and that this effect would not be seen so regularly to follow, if the several organizations did not bear a concerted and contrived relation to the substance by which the animal was surrounded. They would, otherwise, be capacities without objects—powers without employment. The webfoot determines, you say, the duck to swim; but what would that avail if there were no water to swim in? The strong hooked bill and sharp talons of one species of bird determine it to prey upon animals; the soft straight bill and weak claws of another species determine it to pick up seeds; but neither determination could take effect in providing for the sustenance of the birds, if animal bodies and vegetable seeds did not lie within their reach. The peculiar conformation of the bill and tongue and claws of the woodpecker determines that bird to search for his food among the insects lodged behind the bark or in the wood of decayed trees; but what would this profit him if there were no trees, no decayed trees, no insects lodged under their bark or in their trunk? The proboscis with which the bee is furnished determines him to seek for honey; but what would that signify if flowers supplied none? Faculties thrown down upon animals at random, and without reference to the objects amidst which they are placed, would not produce to them the services and benefits which we see; and if there be that reference, then there is intention.
Lastly, the solution fails entirely when applied to plants. The parts of plants answer their uses without any concurrence from the will of the plant.
The existence and character of the Deity is, in every view, the most interesting of all human speculations. In none, however is it more so than as it facilitates the belief of the fundamental articles of revelation. It is a step to have it proved that there must be something in the world more than what we see. It is a further step to know, that among the invisible things of nature there must be an intelligent mind concerned in its production, order, and support. These points being assured to us by natural theology, we may well leave to revelation the disclosure of many particulars which our research cannot reach respecting either the nature of this Being as the original cause of all things, or his character and designs as a moral governor; and not only so, but the more full confirmation of other particulars, of which, though they do not lie altogether beyond our reasonings and our probabilities, the certainty is by no means equal to the importance. The true theist will be the first to listen to any credible communication of divine knowledge. Nothing which he has learnt from natural theology will diminish his desire of further instruction, or his disposition to receive it with humility and thankfulness. He wishes for light, he rejoices in light. His inward veneration of this great Being will incline him to attend with the utmost seriousness, not only to all that can be discovered concerning him by researches into nature, but to all that is taught by a revelation which gives reasonable proof of having proceeded from him.
[ ] indicates portions of the original text were removed
Excerpt from Natural Theology
by William Paley (1802)
다윈을 비롯한 여러 저명한 진화론자들이 페일리의 논증 중 일부를 받아들였다. 어떤 기능을 잘 행하도록 정교하게 생겨먹은 시계나 눈이 “저절로” 생겨났을 리가 없으며 그런 복잡한 구조를 설명하기 위해서는 뭔가 특별한 이야기가 필요하다는 점을 인정한 것이다.
하지만 진화론자들은 그 특별한 설명을 위해서 신이 아니라 자연 선택을 끌어들인다. 진화론자들은 자연 선택이 나온 이후에 페일리 식 “신의 존재 증명”을 받아들이는 것은 시대착오적이라고 이야기한다.
그렇다면 다윈 이전에는 페일리의 논증이 합리적이었나? 그렇게 생각하는 사람들이 꽤 있는 것 같다.
It’s ironic but obvious that until the middle of the nineteenth century teleological explanations in biology were treated as belief/desire explanations of the sort still common in the human sciences. In this case the beliefs and desires that did the explaining were those of God—the omnipotent and benevolent deity who designed everything in the biological realm. What is more, until the nineteenth century the hypothesis that crucial facts about organisms were to be explained in this particularly satisfying way—by appealing to God—was a reasonable one.
Before Darwin’s theory of natural selection, arguably the likeliest explanation for the adaptedness and complexity of biological organization was provided by citing God’s design: one rendered biological organization intelligible by giving the purpose which the organism’s parts play, their roles in God’s plan for the survival and flourishing of the organism. Thus, why does the heart pump the blood? The answer “in order to circulate oxygen” is an abbreviation for something like “God in his goodness wanted to arrange for blood circulation, and unerringly believed that the best way to do this was to build hearts. Since he was omniscient, he was correct in this belief, and since he was omnipotent, he was able to act on it. That is why hearts pump blood.”
(『Philosophy of science: a contemporary introduction』, Alex Rosenberg, 103쪽)
나 역시 시계나 눈의 경우에는 특별한 설명이 필요하다고 생각한다. 하지만 다윈 이전에도 페일리의 논증은 합리적이지 않았다고 생각한다.
만약 “인간이 시계를 만들었듯이 신이 눈을 만들었다”는 식의 페일리의 이야기를 설명(explanation)으로 인정한다면 “인간이 시계를 만들고, 신이 눈을 만들었듯이, 메타신(meta-god)이 신을 만들었다”는 식의 이야기도 설명으로 인정해야 한다.
왜냐하면 눈이 시계보다 더 정교하듯이, (만약 신이 존재한다면) 신이 눈보다 더 정교할 것이 뻔하기 때문이다. 또는, 시계를 만든 인간이 시계보다 더 정교하듯이 인간을 만든 신이 인간보다 더 정교할 것이 뻔하기 때문이다. 따라서 시계나 눈을 위해서 특별한 설명이 필요하다면 신의 존재를 설명하기 위해서도 뭔가 특별한 이야기가 필요하다. 일관성이 있으려면 설계 논증을 신에게도 적용해야 한다.
더 나아가 “인간이 시계를 만들고, 신이 눈을 만들고, 메타신이 신을 만들었듯이 메타메타신(meta-meta-god)이 메타신을 만들었다”는 식의 이야기도 설명으로 인정해야 한다. 그리고 이런 식의 설명(?)은 무한히 계속된다.
뒤집어 이야기하자면 “메타신이 신을 만들었다”는 이야기를 설명으로 인정하지 않는다면 “신이 눈을 만들었다”도 설명으로 인정하지 말아야 한다.
아쉬울 때마다 “신이 그랬다” 또는 “메타신이 그랬다”라고 이야기하는 것은 설명으로 인정할 수 없다. 그렇게 쉽게 설명을 만들어낼 수 있다고 본다면 그것은 세상을 너무 쉽게 살려고 하는 것이다.
신이 인간을 비롯한 온갖 생명을 특별히 창조했다는 식으로 설명하려면 기적 개념이 도입되어야 한다. 여기서 말하는 기적이란 근본적인 물리 법칙이 신의 개입으로 깨지는 순간을 말한다.
Isaac Newton은 1642년에 태어났으며 William Paley는 1743년에 태어났다. 『Natural Theology(자연 신학)』는 1802년에 출간되었으며 이 때는 뉴턴 물리학이 이미 대단한 성공을 거둔 이후였다.
물론 신이 우주를 창조하면서 동시에 인간과 같은 온갖 생명을 창조했으며 그 이후로는 기적이 한 번도 일어나지 않고 근본적인 물리 법칙에 의해 우주가 돌아갔다고 이야기할 수는 있겠다.
신을 끌어들이기 전에 페일리는 물리 법칙만 작동하는 세상에서는 눈과 같은 것이 절대로 생길 수 없다는 것을 입증했어야 했다. 물론 페일리는 그것을 하지 못했다.
페일리는 다만 “내 상상력으로는 도저히 짐작도 할 수 없다”는 식의 생각만 했을 뿐이며 그런 생각에서 “신의 존재”로 직행했다. 페일리가 상상하지 못한다고 해서 길이 없는 것은 아니다. 자신이 상상하지 못하니까 그런 길은 없다고 생각했다면 대단한 과대망상이다.
페일리의 논증을 보고 당시의 과학자가 취했어야 했을 합리적인 입장은 머리를 긁적이면서 “그것 참 희한하네. 왜 그런지 모르겠다”라고 말하는 것이다. 즉 설명해야만 할 문제(눈과 같은 정교한 구조의 존재)가 있음을 인정하면서 그 해답은 모르겠다고 실토하는 것이다.
엉터리 설명을 받아들이는 것보다는 모른다고 말하는 것이 더 합리적이다.