Hume's Dialogues and the Argument from Design

David Hume's Dialogues Concerning Natural Religion involve natural theology, the area of philosophy of religion which seeks to understand the attributes of God purely philosophically, rationally, and scientifically, without recourse to supposed supernatural revelation. Put simply, it seeks to answer the question: what does nature tell us about God? What can we learn about the creator when all we have is access to his creation? A point of weakness of the Dialogues is that "God" is never rigorously defined; I think it goes without saying that there is a supernatural element implied, but other qualities are not universally assumed throughout the Dialogues. In general, we'll find that superlative qualities (like he is almighty, all-loving, of perfect intelligence) cannot be inferred from observations of the natural world, where realizations of these qualities are lacking.

The work is in the form of a dialogue, with three primary characters: Philo, Cleanthes, and Demea. Cleanthes argues that it is possible to understand the attributes of God by reasoning about nature, while Demea's stance is that it is not possible to know God through reason alone. Philo's view is closer to that of Demea than Cleanthes: he argues that inferences about the nature of God based solely on observations of nature are logically incoherent and incomplete, and that any definite conclusions on the matter, of the type argued by Cleanthes, require a priori assumptions about God's character.

My own thoughts tend to align very closely with Philo's professed views; however, my intent in reading the Dialogues was to understand all sides of the contest, and so I tried keep an open mind. By the end, I found Cleanthes' arguments wholly inadequate and unsatisfying, and firmly believe that Philo's view is the only logically sound and rational position.

The Design Argument

The initial thrust of the debate centers on the argument from design, namely, that the resemblance of the universe to a devised machine implies, not only the existence of a designer, but that through analogy with human designers we can come to learn about his nature. This viewpoint was popularized in clergyman William Paley's 1802 watchmaker analogy and still thrives today, fueling the Intelligent Design movement which has achieved recent media attention in several popular legal battles, most notably in Dover, PA school district [1]. Hume's Dialogues present one of the earliest critical accounts of this appeal, and Philo convincingly destroys any vestige of validity it might be thought to contain. Cleanthes invokes the design argument in order to assert that the "author of nature is similar to the mind of man". Regarding the world, Cleanthes states (p. 15):

You will find it to be nothing but one great machine, subdivided into an infinite number of lesser machines... All these various machines, and even their most minute parts, are adjusted to each other with an accuracy which ravishes into admiration all men who have ever contemplated them. The curious adapting of means to ends, throughout all nature, resembles exactly, though it much exceeds, the productions of human contrivance; of human design, thought, wisdom, and intelligence.

He then concludes, crucially,

Since therefore the effects resemble each other, we are led to infer, by all the rules of analogy, that the causes also resemble, and that the Author of Nature is somewhat similar to the mind of man...

This same reasoning has found voice in the modern Intelligent Design adage, "if it walks like a duck and talks like a duck..." leaving the unfortunate listener to conclude for himself that the uncanny resemblance of the eyeball to the camera alone decrees design at the highest echelon of the supernatural. There are two obvious problems with Cleanthes' claim: 1) it's only an analogy, far from a logical proof, and 2) it's a pretty bad analogy. Philo counters (p. 16)

If we see a house, Cleanthes, we conclude, with the greatest certainty, that it had an architect or builder because this is precisely that species of effect which we have experienced to proceed from the species of cause.

Indeed. If we happen upon a cabin in the woods, we can, with great confidence, assert that it is the product of human invention without further investigation. The same degree of confidence, however, does not follow for natural objects because this evidence is distinctly lacking -- hence the need for analogy. The analogy, it turns out, is a poor one, as Philo continues:

But surely you will not affirm that the universe bears such a resemblance to a house that we can with the same certainty infer a similar cause, or that the analogy is here entire and perfect. The dissimilitude is so striking that the utmost you can here pretend to is a guess, a conjecture, a presumption concerning a similar cause...

Cleanthes disagrees that his argument amounts to no more than conjecture or presumption, challenging Philo to recognize the resemblance between a house and the universe in terms of the "order, proportion, and arrangement of every part." He offers the following example in elaboration (p. 16),

Steps of a stair are plainly contrived that human legs may use them in mounting; and this inference is certain and infallible. Human legs are also contrived for walking and mounting; and this not altogether so certain because of the dissimilarity which you remark; but does it therefore deserve the name only of presumption or conjecture?

Yes, it does, because the dissimilarity is manifest -- one is an inanimate object for which we have direct proof of design; the other is an organism altogether different from stairs made of wood or houses of stone. Humans are living beings with the ability to grow, learn, react to stimuli -- in effect, they are like nothing ever observed to be the product of any known design principle1
Mechanical devices with artificial intelligence sufficient to pass the Turing Test notwithstanding. But assuming man is capable of creating such devices makes the question of what it takes to create human life perhaps more tractable.
. And Cleanthes intends for this design principle to extend beyond humans, to encompass the whole of the universe -- the rocks, the air, the darkness of space, which in and of themselves, don't do anything.

We can grant Cleanthes a bit of a pass here, because at the time the Dialogues was written, human understanding of man's origins and the nature of the universe was the stuff of mythology and astrology. But the crazy thing is that there are modern-day proponents of the design principle advanced by Cleanthes, despite the vast leaps in knowledge that mankind has achieved on the frontiers of science. Against this modern backdrop of a much advanced understanding of the world, the design argument faces a steep uphill climb.

Cleanthes argues that both life and non-life are part of God's blueprint. I think it can be convincingly shown that neither is, and that the appearance of design in the universe is illusory. Apparent design can arise in several ways: through a natural, algorithmic process that works to optimize a subject's form and function given its environment, through the requirement that the environment be suitable for life, and through chance or other constraints.

We have substantial evidence that the first applies to living organisms with heritable traits in resource-constrained environments. This process of evolution through natural selection results in organisms that are specialized, or tuned, to their surroundings, with adaptations greatly resembling the characteristics of a well-conceived design. But are they the characteristics expected of the high-quality craftsmanship of supernatural agency? We don't expect so. While natural selection works to render organisms in some sense "optimized" to their surroundings, it does so via blind twists and turns and with certain limitations that sometimes make for an inefficient or otherwise kluged adaptation. A mutation responsible for an imperfect adaptation that "did the trick" might remain in a species' genome, a consequence of the unguided shuffling of traits that form the genetic bedrock of Darwinian evolution. Humans alone possess countless features that betray this dysteleogical Darwinian slog: backwards retinas, precariously laid ureters, narrow hips, a tiny birth canal, a shared tube for eating and breathing (the simultaneous action of which results in certain death), and, to channel Neil DeGrasse Tyson, "an entertainment complex built around a sewage system".

Fig. 1 Some flaws of the human physical apparatus, and proposed fixes. The point is that a designer intent on building a durable and efficient device would have crafted humans to look more like the form on the right. Click for a larger view. From "If Humans Were Built to Last" [2].

Following the view of Gould and Lewontin, not all biological traits are necessarily even adaptations honed directly by evolution. In their seminal paper (with the super-pretentious title), "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" [3], Gould and Lewontin provide examples of phenotypes that are essentially byproducts of adaptations, that, despite appearances to the contrary, are not themselves direct products of adaptive selection. Examples abound in evolutionary biology, but as per the title of the paper, we see the same sort of thing in architecture. A spandrel is the space between two arches; if these arches happen to be supporting a hemispherical dome, the spandrel is a symmetric region in the shape of a triangle with arced sides, perfect for artistic or architectural design elements. Naively, the spandrel appears planned because it so naturally accents the surrounding structure, but really, it couldn't be any other way!2
Philosopher Dan Dennett objects, citing alternative structures that could have been used in place of the spandrel; but, the spandrel still succeeds as a proof of concept.
. So, as design can be feigned in human-devised structures, so too can apparent design arise in biological organisms, apart from true adaptations that might equally suggest design.

Fig. 2 Spandrel. Credit: McGraw-Hill Dictionary of Architecture and Construction.

Cleanthes argues that non-life, too, might be the result of careful design, citing the "order" and "arrangement" seen in nature. These qualities are generally associated with nature's favorability to life, especially complex life like humans. But is the universe especially conducive to human existence and flourishing? I've never been to the event horizon of a black hole, or the magnetic shit storm that lies at the gates of a neutron star, much less the dark, cold, empty surface of a lonely asteroid, but these places are undeniably hostile to life. With nothing more than a backyard telescope, we can gaze upon our celestial neighbors -- lifeless (the possibility of microbial life on certain moons in the solar system notwithstanding, the focus here is complex, sapient life) -- at the extremes of temperature and chemistry: from searing Venus enrobed in thick, poisonous clouds to lonely Saturn, with core densities sufficient to turn hydrogen gas into metal, and wind speeds upwards of 1000 miles per hour. Simple accounting reveals that these lifeless locales are vastly more numerous in the universe than our tiny speck of habitat. A design principle based on wasted space, much of which is directly hostile to a key component of the design scheme (humans), which are themselves poorly devised meatbags riddled with imperfection, is the design principle of a madman or a dunce. If Cleanthes hopes to infer the character of God from such creations under the presumed analogy with humans, he is forced to conceive of a deity either insane or inept.

But, there are, after all, places in the universe where life thrives. Earth comes to mind as the only place discovered so far to harbor sapient life. But it does so only across a narrow band of conditions -- not too hot, not too cold, only just the right amount of rain and wind, air pressure, oxygen. Adverse environmental conditions aside, there are also bears that wish to rend you limb from limb, viruses that seek to turn your insides into mush, and meteors that occasionally obliterate vast swaths of the planet's inhabitants. In this light, one view might be that life thrives on Earth despite conditions that work directly against it, vitiating any design argument.

Some, however, particularly those of religious persuasion, are moved by the singular beauty of life and Earth's seemingly exclusive ability to support it. After all, despite the dangers of the world and the fragility of life, humans have thrived for thousands of years on this planet. And doesn't the world appear strikingly ordered? To quote Bill O'Reilly, "You know, I just don't think we could've lucked out to have the tides come in, the tides go out, sun go up, sun go down. Don't think it could've happened." [4] Lucked out? I admit I don't know what he means, but he appears to be arguing that conditions on Earth are simply too perfect -- too ordered and awe-inspiring -- to be chalked up to luck. Is there something to this line of argument?

Sort of. In reality, some things are almost certainly chance occurrences without any significant bearing on human existence. The rhythm of the tides, the ruddy glow of the sunset, the smell of a summer storm, all in some sense appeal to our aesthetic and suggest a deeper importance, but are apparently only incidental to our life on Earth. The beauty of these things is subjective, and not at issue. The questionable step is taken when this sense of beauty is interpreted as indicating something purposeful. They may well be, but it's more likely that this misattribution follows from the fact that humans are pattern-seeking mammals, and wildly successful at it.

But some things really are necessary. They are not, however, the result of some divine plan; they occur because they must, because they could not be otherwise! This is the anthropic selection argument -- the physical universe must have properties compatible with the existence of beings fit to observe these properties. In this light, the Earth is indeed special, in that it happens to possess the necessary conditions. But life didn't thrive on Earth by design -- it arose simply because it could. The anthropic principle is often misused by religious apologists (I'm looking at you Dinesh D'Souza), who claim that it supports the idea that conditions on Earth are perfectly tuned to support life. It turns out they are, but nobody's doing the tuning.

While special in the sense that the conditions needed to support human existence on Earth are a razor-thin slice through the menagerie of all possible conditions, they are truly arbitrary from the standpoint of the universe. It is hard for humans not to be prejudicial about the necessary conditions and ingredients for life, since we naturally presume that they must conform to our experience; however, there is nothing a priori exclusive about carbon-based life, and humans are unlikely to be the only way that intelligent life exhibits itself. The fact that organisms thrive in Earth's most extreme environs, from roiling undersea thermal vents to frigid Antarctic permafrost, should weaken our surety of just what the "right" conditions for life are. Considering in the theoretical abstract all the different biochemistries capable of supporting life, and the structural and neurological systems they might compose, it is supremely foolish to suppose that the comparatively temperate climes of the Earth are unique to complex, intelligent life. Such claims betray a failure of imagination, and they place into stark relief the fallacy of our impressions that perfect balance and order are necessary for existence. The anthropic argument, therefore, effectively demonstrates two things: that conditions on Earth only appear specially-tuned because they must be that way in order for human life to emerge; and that these conditions are truly arbitrary if one enlarges the pallet of color from which life can be composed. The anthropic argument does not prove that human life just happened wherever it could in the course of the mechanical evolution of the universe, but it provides a simple plausibility argument that this is almost inevitably so, given that physics works the way it does.3
This line of argument assumes that life originated through a strictly physical process so that all that is needed are the right physical conditions; this is the assumption underlying the Dialogues as the discussion centers on natural religion.

Philo summarizes the spirit of the anthropic argument (p. 57)

Is it not probable...that the whole economy of the universe is conducted by a like necessity, though no human algebra can furnish a key which solves the difficulty? And instead of admiring the order of natural beings, may it not happen that, could we penetrate into the intimate nature of bodies, we should clearly see why it was absolutely impossible they could ever admit of any other disposition?

Ultimately, our sense of life's aesthetics and our deeply-felt compatibility with the Earth are consequences of the fact that this is the only life we know. A poet living on Titan could find heaven in the beauty of acrid oceans of ethane lapping frozen acetylene shores; Earthlings accustomed to the calm, sweet breezes of the Caribbean would find this the prototype of hell frozen over.

It is possible, at this point, to raise an important objection to the preceding analysis. I've countered the design argument by citing examples of natural imperfection that belie any impression of careful design. But who are we to judge this workmanship?

There are many inexplicable difficulties in the works of Nature, which, if we allow a perfect author to be proved a priori, are easily solved, and become only seeming difficulties from the narrow capacity of man, who cannot trace infinite is impossible for us to tell, from our limited views, whether this system contains any great faults or deserved any considerable praise if compared to other possible and even real systems. Could a peasant, if the Aeneid were read to him, pronounce that poem to be absolutely faultless...?

It certainly seems presumptuous and arrogant of mankind to judge the talents and merits of a designer based on what might be only a partial glimpse of his product. In leveling our verdict, we had in mind a purpose towards which this creation was intended to act, and finding it inefficient and ineffective at achieving these goals, declared it of poor design. It is possible that some cosmic supernatural intelligence has intentions far beyond the grasp of human understanding, that he is appealing to an aesthetic beyond our comprehension, or even that he has a pernicious plan indifferent to human toil. But this deflates whatever power the design argument might have had. Supposing that a designer deliberately made something of poor design, or that he was appealing to an aesthetic beyond our appreciation, would belie the impression of design allegedly apparent in nature that motivated the design argument in the first place! Without a basis for comparison and ignorant of any ultimate purpose, our opinions on God's creation are meaningless.

Tortoises, all the way down

In A Brief History of Time, Stephen Hawking relates an amusing anecdote:

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist gave a superior smile before replying, "What is the tortoise standing on?" "You're very clever, young man, very clever," said the old lady. "But it's tortoises all the way down!"

Suppose that we grant Cleanthes his designer. If we continue this line of argument, using observations of the natural world to infer the characteristics of this designer, we quickly run up against the following observation: like material intelligence, the mind of God must also be subjected to cause and effect. This inference is a direct result of Cleanthes anthropomorphism -- his assertion that the character of God is analogous to that of man. Philo argues (p. 31):

How, therefore, shall we satisfy ourselves concerning the cause of that Being whom you suppose the Author of Nature, or, according to your system of anthropomorphism, the Ideal World into which you trace the material [world]? Have we not the same reason to trace that ideal world into another ideal world or new intelligent principle? But if we stop and go no farther, why go so far? Why not stop at the material world? How can we satisfy ourselves without going on in infinitim?

The quest for the world's ultimate cause is book-ended by absurdity: either we arbitrarily stop somewhere along the causal chain, or we go along it forever. If we take the anthropomorphic analogy at its fullest, then we are compelled to search for a cause of God himself: if man is caused by something (God), and if God is anthropomorphic, then God is caused by something. Cleanthes defiantly dismisses the problem (p. 32), ask me what is the cause of the cause? I know not; I care not; that concerns me not. I have found a Deity, and here I stop by inquiry.

That's bullshit. The word "Deity" at this point in the dialectic equates to "the creator of the universe". Even if we make Cleanthes this concession, we still don't really know anything else about this creator -- whether he's loving, hateful, eternal; all we know is that he is to be similar to man in his devices, although presumably more impressive in his abilities (you know, creating entire universes while man toils in meth labs here on Earth). We are not justified in elevating him to the ultimate cause of all causes, or to raise him above physical causality into the supernatural -- we can't use semantics to break out of the bounds of his presumed anthropomorphism. Cleanthes' lack of concern about this problem is the epitome of intellectual laziness.

Fig. 3 Turtles.

If the complex intelligence of the deity requires no stated cause -- if it can just be -- then, echoing Philo, why go so far? Why not simply presume that the natural world itself can just be? Any supreme intelligence is at least as complex, ordered, and perfect as its creation, and so postulating the existence of an uncaused supreme intelligence as the cause of a less perfect natural world is not only logically unnecessary, it is an astoundingly overcomplicated explanation of the natural world. In short, it needs a shave from Occam's razor. Philo puts this succinctly (p. 33),

An ideal system, arranged of itself, without a precedent design, is not a whit more explicable than a material one which attains its order in a like manner...

Bertrand Russell hits on the same point in Why I am not a Christian, "If everything must have a cause, then God must have a cause. If there can be anything without a cause, it may just as well be the world as God, so that there cannot be any validity in that argument. It is exactly of the same nature as the Hindu's view, that the world rested upon an elephant and the elephant rested upon a tortoise; and when they said, 'How about the tortoise?' the Indian said, 'Suppose we change the subject.'"

While the unmoved mover may not be a logical impossibility, we can't learn anything about it through observation because there is nothing in the material world like it -- not even in principle.

The Burden of Proof

By now there is a clear, insurmountable difficulty of Cleanthes' position. He is trying to infer something about the supernatural by observing the natural -- two non-overlapping regimes. In so doing, he is condemning his deity to the drab world of material existence, one devoid of magic and perfection, and yet he wants his deity to enjoy these privileges. Cleanthes has a conception of God -- a singular entity of perfection that is all-powerful -- that is in no obvious way informed by observations of the natural world; as a result, the burden of proof falls squarely on Cleanthes. Philo raises the point (pp. 35-36),

You have no reason...for ascribing perfection of the Deity, even in his finite capacity; or for supposing him free from every error, mistake, or incoherence, in his undertakings...And what shadow of argument can you produce from your hypothesis to prove the unity of the Deity? A great number of men join in building a house or a ship, in rearing a city, in framing a commonwealth...

Even the seemingly easy supposition that there is one creator is not something inferable from any Earthly observation, and yet, man is evidently accustomed to this monotheistic view. In short, nature offers virtually no guidance on the question,

Without any great effort of thought...I could propose other systems of cosmogeny which would have some faint appearance of truth; though it is a thousand, a million to one if either yours or any one of mine be the true system.

Like Russell's celestial teapot purportedly in orbit somewhere between Earth and Mars, there are many potential physical realities that are apparently consistent with the natural world. If it is true that religion is a man-made enterprise, it should come as no surprise that the religious texts prescribe a world compatible with that around us. This is a kind of construction ex post facto. As a result, at best one can expect religious accounting that is consistent with the world as it is, and the foregoing difficulty arises because there is a near infinitude of creation stories, accounts of cosmogenesis, and types of gods that are each consistent with the world as known by man. The logical induction from observation is a one-to-many mapping: ultimately any conception of God can be devised. One might be as inclined to postulate the existence of an exalted, omnipotent ball of pasta, rather than the generic visage of an old bearded man; or maybe the Deity is more like Zeus after all than the God of Abraham. In such cases, the burden of proof is on those who advance the truth of a particular religious system or Deity over the others.

Fig. 4 Potential deities: (a) Vampire, Bill Compton (b) The Flying Spaghetti Monster (c) Magical Lion, Aslan (d) Egyptian Bird-God, Horus.

Philo argues (p. 68),

But supposing, which is the real case with regard to man, that this creature is not antecedently convinced of a supreme intelligence, benevolent, and powerful, but is left to gather such a belief from the appearances of things; this entirely alters the case, nor will he ever find any reason for such a conclusion.

My wife refers to the above difficulty as the creationists' "missing link": where is the empiricist that, through nothing more than observations of the natural world, inferred that it was created in six days, that there was a virgin birth, a resurrection, a heaven and hell? In reality, no honest person makes these conclusions given the material evidence.

It is also the case that many religious accounts of the world are embarrassingly inconsistent with human experience. This was less true when the canonical religious texts are said to have been written; if the creation stories and other claims about natural history are myth, the writer could maintain consistency with the world around him to any desired degree. There are many examples in the Old Testament of assertions of fact whose veracity at the time of writing was unknown, that have only in comparatively recent times been revealed to be in conflict with human understanding. These claims tend to refer to the scientifically measurable facets of reality, like the age of the cosmos, the geometry of the solar system, the process by which living beings came to populate the Earth, including the conspicuously incomplete list of beasts created by God on the 6th day, a list that is nonetheless consistent with the fauna endemic to the writer's land at the time. In general, the consistency and veracity of religious mythos are belied by the finite time and space of its creators.

While the process of induction is inadequate for declaring a one true theology, we can still deductively falsify statements: those parts of a system of a religion that make claims about the natural world must pass experimental muster. A claim that the Earth is on or about 6000 years old can be decisively falsified by measuring this age empirically: at best, the claims are deemed consistent, at worst, they are found contradictory.

The Best of All Possible Worlds

The Dialogoues segue from the question of design to that of morality. In keeping with Cleanthes' anthropomorphic conception of God, Philo suggests the "moral attributes of the Deity, his justice, benevolence, mercy, and rectitude, to be of the same nature with these virtues in human creatures." (p. 63) The idea is that the manifestations of good and evil in the world might shed light on the question of God's benevolence, malevolence, or ambivalence.

Now, a despot who deliberately suppresses and terrorizes his subjects is deemed evil, while a loving and virtuous ruler ensures the peace and happiness of his followers. If God is to be like man in his moral inclinations, then this same reasoning applies. Philo's position is that there is much pain, strife, and misery in the world. How are we to judge God? Philo recapitulates Epicurus (p. 63)

Is he willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? whence then is evil?

It is not enough to observe the human condition: we must also understand the will and degree of control that God has over the world. This is not something that we can induce from observation; however, we can propose and follow various starting assumptions to their logical end. It is proposed at about this point that God be considered almighty and perfectly benevolent. (This is a fine hypothesis, but it is hardly consistent with the anthropomorphism that has formed the basis of much of the discussion thus far. A small perturbation on man's character might be allowed the Deity, who we tacitly assume is "like" man but better at certain things. But augmenting a subject with infinite amounts of anything so drastically changes the subject that a comparison is likely impossible; if man is summed up by his accomplishments and his limitations, an infinitely powerful entity has a superlative and limitless resume. There is nothing human about it.) Cleanthes and Philo spend a good deal of time arguing whether the world is perfectly good, unfortunately dismal, or somewhere in between. Cleanthes maintains that, surely, if one were to tally up all the pains and joys in the world, the joys would have it (p. 64). Philo, correctly, counters that such an exact accounting is unnecessary (p. 66),

No decisive proofs can ever be produced against this authority; nor is it possible for you to compute, estimate, and compare all the pains and all the pleasures in the lives of all men and of all animals...But allowing you [this], this is not, by any means, what we expect from infinite power, infinite wisdom, and infinite goodness. Why is there any misery at all in the world?...Is it from the intention of the Deity? But he is perfectly benevolent. Is it contrary to his intention? But he is almighty.

Philo concludes that an understanding of the subject exceeds all human capacity, and that our logic simply doesn't apply. A much more reasonable position is simply that the premises of infinite power and perfect benevolence are false. So, no, evidently not the best of all possible worlds. But even if it were, we'd be powerless to infer benevolence without first postulating God's will and ability to bless our world.


At this point, we must part ways with Philo. In the final dialogue, Philo unexpectedly cedes the argument to Cleanthes, admitting, after all, that the world obviously appears designed4
Philo's complete reversal is not understood within the context of the Dialogues. It is thought that Hume sought to temper Philo's position to avoid any adverse reactions to his skepticism of theism.
. As argued, this is a myopic and unsatisfying view that fails to provide adequate empirical support to natural theology. Philo does conclude, correctly I believe, that regardless of any impression of design in the universe, the only true way to worship God is through revelation and, ultimately, faith.

Stephen J. Gould spoke of "non-overlapping magisteria", referring to the mutual exclusivity of religious and scientific domains of thought. Natural theology, the focus of the Dialogues, would provide an important counterexample of this notion if found to be a coherent system by which one could know God. Hume, through Philo, shows decisively that the kind of natural theology argued by Cleanthes is a failure. What is interesting, to me, is that natural theology is still vigorously defended, even today, by religious apologists and popularizers. The question of God's existence has become an actual debate topic, where people attempt a resolution through reason, citing the epistemic reaches of science, or claiming to have found evidence for God in the cosmic microwave background. This is an errant line of inquiry guaranteed to end in exasperation and without resolution, and the reason is simple.

The reason is simple. God's existence: his ways, workings, qualities, are either physical or not. If they are not physical -- if they transcend or supersede physical law -- then they might not be testable. For example, consider the claim that God controls our universe and our destiny: if this mechanism of control exists outside our physical universe, it cannot be tested through any means within our physical universe. In this case, the only way to understand God is through faith, because his presence in our lives is not verifiable. This faith is essentially blind, because it is uninformed by evidence. If, on the other hand, God's ways, working, and qualities are physical, that is, occurring in physical space according to physical law, then they are in principle empirically falsifiable. This means that we can use science as a tool to investigate God as a natural phenomenon. This is essentially the basis of natural theology. The above distinction is what Gould had in mind with the non-overlapping magisteria -- the natural and the supernatural can coexist independent of one another, as two orthogonal domains.

Does this weaken the case for God's existence? Hardly. But, it does reveal an important difficulty in the reconciliation of religion and science, and that is that belief in the supernatural requires that we assume as true something ultimately unverifiable. Meanwhile, science trains us to relentlessly challenge the truth of our assumptions. This is not to say that physicalism prevails and that the material world is all there is, but it argues strongly that questions of God, and the supernatural more generally, are not subjects of scientific enquiry, but questions of faith.

[2] Olchansky, Carnes, Butler, "If Humans Were Built to Last", Scientific American, 2001. Link.
[3] Gould, Lewontin, "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme", Proceedings of the Royal Society of London, Series B, Vol. 205, No. 1161 (1979). Link to paper.

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