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For a long time, then, I reflected on this confusion in the astronomical traditions concerning the derivation of the motions of the universe's spheres. I began to be annoyed that the movements of the world machine, created for our sake by the best and most systematic Artisan of all, were not understood with greater certainty by the philosophers, who otherwise examined so precisely the most insignificant trifles of this world. For this reason I undertook the task of rereading the works of all the philosophers which I could obtain to learn whether anyone had ever proposed other motions of the universe's spheres than those expounded by the teachers of astronomy in the schools. And in fact first I found in Cicero that Hicetas supposed the earth to move. Later I also discovered in Plutarch that certain others were of this opinion. I have decided to set his words down here, so that they may be available to everyone: "Some think that the eart remains at rest. But Philolaus the Pythagorean believes that, like the sun and moon, it revolves around the fire in an oblique circle. Heraclides of Pontus and Exphantus the Pythagorean make the earth move, not in a progressive motion, but like a wheel in a rotation from the west to east about its own center."

Research men attempt to satisfy their curiosity, and are accustomed to use any reasonable means that may assist them toward the receding goal. One of the few universal characteristics is a healthy skepticism toward unverified speculations. These are regarded as topics for conversation until tests can be devised. Only then do they attain the dignity of subjects for investigation. ... With increasing distance our knowledge fades and fades rapidly. Eventually we reach the dim boundary, the utmost limits of our telescope. There we measure shadows, and we search among ghostly errors of measurements for landmarks that are scarcely more substantial. The search will continue. Not until the empirical resources are exhausted need we pass on to the dreamy realms of speculation.

When it is asked what we ought to believe in matters of religion, the answer is not to be sought in the exploration of the nature of things, after the manner of those whom the Greeks called "physicists". Nor should we be dismayed if Christians are ignorant about the properties and the numbers of the basic elements of nature, or about the motion, order, and deviations of the stars, the map of the heavens, the kinds and nature of animals, plants, stones, springs, rivers, and mountains; about the divisions of space and time, about the signs of impending storms, and the myriad other things which these "physicists" have come to understand, or think they have. For even these men, gifted with such superior insight, with their ardor in study and their abundant leisure, exploring some of these matters by human conjecture and others through historical inquiry, have not yet learned everything there is to know. For the Christian it is enough to believe that the cause of all created things, whether in heaven or on earth, whether visible or invisible, is nothing other than the goodness of the Creator, who is the one and the true God.

Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of this world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he holds as certain from reason and experience. Now it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn.

In a similar way we may conceive that progress may be made in natural theology in either of two ways: by deducing consequences from what we know or observe, or by assuming for trial the truth of a statement made on whatever authority it may be, and then examining whether the supposition of its truth so falls in with such knowledge as we possess, or such phenomena as we observe, as to lead us to a conviction that the statement does indeed express the truth. It may be that the statement comes from a source which professes to be a revelation made from God to man. But such an employment of it as I have just described is strictly analogous to our procedure in the study of physical science, and does not therefore seem to be precluded by the terms of the foundation of this lectureship.

In science itself I certainly want to include the farthest flights of physics and cosmology, as well as experimental psychology, history, and the social sciences. Also, mathematics, insofar at least as it is applied, for it is indispensable to natural science. What then am I excluding as "some prior philosophy," and why? Descartes' dualism between mind and body is called metaphysics, but it could as well be reckoned as science, however false. He even had a causal theory of the interaction of mind and body through the pineal gland. If I saw indirect explanatory benefit in positing sensibilia, possibilia, spirits, a Creator, I would joyfully accord them scientific status too, on a par with such avowedly scientific positions as quarks and black holes. What then have I banned under the name of prior philosophy? ¶ Demarcation is not my purpose. My point in the characterization of naturalism ... is just that the most we can reasonably seek in support of an inventory and description of reality is testability of it observable consequences in the time-honored hypothetico-deductive way — whereof more anon. Naturalism need not cast aspersion on irresponsible metaphysics, however deserved, much less on soft sciences or on the speculative reaches of the hard ones, except insofar as a firmer basis is claimed for them than the experimental method itself.

So yes, my arguments might give us reason to prefer natural explanations when these are available, and to seek natural explanations when they are not. It follows that a proposed theistic explanation should be, at best, an explanation of last resort. One might argue that this view — that we should abandon the search for natural explanations only in extremis — represents a kind of "presumption of naturalism." And so it does. ¶ My own view is that the naturalistic research tradition of the sciences has been stunningly successful and must rank as one of the greatest of human achievements. But I think it is poorly served by attempts to define science in such a way as to exclude the supernatural. The debate over intelligent design is instructive in this regard. One might win a legal victory by insisting that this proposed theistic explanation is not what we customarily call "science." And this is true, for contingent historical reasons. But it would be much more effective to show that this particular proposed theistic explanation, with its deliberately vague appeal to an unspecified "designer," is practically vacuous. it lacks the first and most important virtue of any proposed explanation, namely that of testability. It follows that this particular proposed theistic explanation should be rejected. ¶ Could the theist produce a better one? I doubt it, but then it would be most regrettable if we were to forbid him to try. Nothing could be more antithetical to the spirit of free enquiry than this kind of censorship. If proposed theistic explanations are to be defeated, as they have been so often in the past, it will be by way of the free contest of ideas.

There is a crucial difference between these two cases. All other integrations into a super-science, of sciences dealing with entities and properties apparently qualitatively very distinct, was achieved by saying that really some of the entities and properties were not as they appeared to be; by making a distinction between the underlying (not immediately observable) entities and properties and the phenomenal properties to which they give rise. Thermodynamics was conceived with the laws of temperature exchange; and temperature was supposed to be a property inherent in an object. The felt hotness of a hot body is indeed qualitatively distinct from particle velocities and collisions. The reduction was achieved by distinguishing between the underlying cause of the hotness (the motion of the molecules) and the sensations which the motion of molecules cause in observers. The former falls naturally within the scope of statistical mechanics — for molecules are particles; the entities and properties are not now of distinct kinds. But this reduction has been achieved at the price of separating off the phenomenal from its causes, and only explaining the latter.

There is such proneness in men of genius to invent hypotheses, and in others to acquiesce in them as the utmost which the human faculties can attain in philosophy, that it is of the last consequence to the progress of real knowledge, that men should have a clear and distinct understanding of the nature of hypotheses in philosophy, and of the regard that is due to them. ¶ Although some conjectures may have a considerable degree of probability, yet it is evidently in the nature of conjecture to be uncertain. In every case, the assent ought to be proportioned to the evidence; for to believe firmly what has but a small degree of probability, is a manifest abuse of our understanding. Now, though we may, in many cases, form very probable conjectures concerning the works of men, every conjecture we can form with regard to the works of God has as little probability as the conjectures of a child with regard to the works of a man.

From time to time various writers have told us that we cannot reach any conclusions about the origin or development of the universe, since it is (whether by logic or just in fact) a unique object, the only one of its kind, and rational inquiry can only reach the conclusions about objects which belong to kinds, e.g. it can reach a conclusion about what will happen to this bit of iron, because there are other bits of iron, the behaviour of which can be studied. This objection of course has the surprising, and to most of these writers unwelcome, consequence, that physical cosmology cannot reach justified conclusions about such matters as the size, age, rate of expansion, and density of the universe as a whole (because it is a unique object); and also that physical anthropology cannot reach conclusions about the origin and development of the human race (because, as far as our knowledge goes, it is the only one of its kind). The implausibility of these consequences leads us to doubt the original objection, which is indeed misguided.