In the history of ideas, there are examples of questions being answered that had earlier been judged forever out of science's reach. In 1835 the celebrated French philosopher Auguste Comte wrote, of the stars: 'We shall never be able to study, by any method, their chemical composition or their mineralogical structure.' Yet even before Comte had set down these words, Fraunhofer had begun using his spectroscope to analyse the chemical composition of the sun. Now spectroscopists daily confound Comte's agnosticism with their long-distance analyses of the precise chemical composition of even distant stars.29 Whatever the exact status of Comte's astronomical agnosticism, this cautionary tale suggests, at the very least, that we should hesitate before proclaiming the eternal verity of agnosticism too loudly. Nevertheless, when it comes to God, a great many philosophers and scientists are glad to do so, beginning with the inventor of the word itself, T. H. Huxley.30
Huxley explained his coining while rising to a personal attack that it had provoked. The Principal of King's College, London, the Reverend Dr Wace, had poured scorn on Huxley's 'cowardly agnosticism':
He may prefer to call himself an agnostic; but his real name is an older one — he is an infidel; that is to say, an unbeliever. The word infidel, perhaps, carries an unpleasant significance. Perhaps it is right that it should. It is, and it ought to be, an unpleasant thing for a man to have to say plainly that he does not believe in Jesus Christ.
Huxley was not the man to let that sort of provocation pass him by, and his reply in 1889 was as robustly scathing as we should expect (although never departing from scrupulous good manners: as Darwin's Bulldog, his teeth were sharpened by urbane Victorian irony). Eventually, having dealt Dr Wace his just comeuppance and buried the remains, Huxley returned to the word 'agnostic' and explained how he first came by it. Others, he noted,
were quite sure they had attained a certain 'gnosis' — had, more or less successfully, solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble. And, with Hume and Kant on my side, I could not think myself presumptuous in holding fast by that opinion… So I took thought, and invented what I conceived to be the appropriate title of 'agnostic'.
Later in his speech, Huxley went on to explain that agnostics have no creed, not even a negative one.
Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle. Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable. That I take to be the agnostic faith, which if a man keep whole and undefiled, he shall not be ashamed to look the universe in the face, whatever the future may have in store for him.
To a scientist these are noble words, and one doesn't criticize T. H. Huxley lightly. But Huxley, in his concentration upon the absolute impossibility of proving or disproving God, seems to have been ignoring the shading of probability. The fact that we can neither prove nor disprove the existence of something does, not put existence and non-existence on an even footing. I don't think Huxley would disagree, and I suspect that when he appeared to do so he was bending over backwards to concede a point, in the interests of securing another one. We have all done this at one time or another.
Contrary to Huxley, I shall suggest that the existence of God is a scientific hypothesis like any other. Even if hard to test in practice, it belongs in the same TAP or temporary agnosticism box as the controversies over the Permian and Cretaceous extinctions. God's existence or non-existence is a scientific fact about the universe, discoverable in principle if not in practice. If he existed and chose to reveal it, God himself could clinch the argument, noisily and unequivocally, in his favour. And even if God's existence is never proved or disproved with certainty one way or the other, available evidence and reasoning may yield an estimate of probability far from 50 per cent.
Let us, then, take the idea of a spectrum of probabilities seriously, and place human judgements about the existence of God along it, between two extremes of opposite certainty. The spectrum is continuous, but it can be represented by the following seven milestones along the way.
1 Strong theist. 100 per cent probability of God. In the words of C. G. Jung, 'I do not believe, I know.'
2 Very high probability but short of 100 per cent. Defacto theist. 'I cannot know for certain, but I strongly believe in God and live my life on the assumption that he is there.'
3 Higher than 50 per cent but not very high. Technically agnostic but leaning towards theism. 'I am very uncertain, but I am inclined to believe in God.'
4 Exactly 50 per cent. Completely impartial agnostic. 'God's existence and non-existence are exactly equiprobable.'
5 Lower than 50 per cent but not very low. Technically agnostic but leaning towards atheism. 'I don't know whether God exists but I'm inclined to be sceptical.'
6 Very low probability, but short of zero. De facto atheist. 'I cannot know for certain but I think God is very improbable, and I live my life on the assumption that he is not there.'
7 Strong atheist. 'I know there is no God, with the same conviction as Jung "knows" there is one.'
I'd be surprised to meet many people in category 7, but I include it for symmetry with category 1, which is well populated. It is in the nature of faith that one is capable, like Jung, of holding a belief without adequate reason to do so (Jung also believed that particular books on his shelf spontaneously exploded with a loud bang). Atheists do not have faith; and reason alone could not propel one to total conviction that anything definitely does not exist. Hence category 7 is in practice rather emptier than its opposite number, category 1, which has many devoted inhabitants. I count myself in category 6, but leaning towards 7 — I am agnostic only to the extent that I am agnostic about fairies at the bottom of the garden.
The spectrum of probabilities works well for TAP (temporary agnosticism in practice). It is superficially tempting to place PAP (permanent agnosticism in principle) in the middle of the spectrum, with a 50 per cent probability of God's existence, but this is not correct. PAP agnostics aver that we cannot say anything, one way or the other, on the question of whether or not God exists. The question, for PAP agnostics, is in principle unanswerable, and they should strictly refuse to place themselves anywhere on the spectrum of probabilities. The fact that I cannot know whether your red is the same as my green doesn't make the probability 50 per cent. The proposition on offer is too meaningless to be dignified with a probability. Nevertheless, it is a common error, which we shall meet again, to leap from the premise that the question of God's existence is in principle unanswerable to the conclusion that his existence and his non-existence are equiprobable.
Another way to express that error is in terms of the burden of proof, and in this form it is pleasingly demonstrated by Bertrand Russell's parable of the celestial teapot.31
Many orthodox people speak as though it were the business of sceptics to disprove received dogmas rather than of dogmatists to prove them. This is, of course, a mistake. If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time.