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The dangers of reverse-reasoning: a Christmas parable

December 23rd, 2017

Beware starting from a conclusion and working backwards

“This time next year, we’ll be running the country”, as Jeremy Corbyn didn’t quite say a few days ago in his interview with Grazia. It’s a near-repetition of his prediction at Glastonbury this June – except that there he was talking about Christmas 2017 rather than 2018 – and for those not favourably inclined towards him, might bear a passing resemblance to the unsubstantiated optimism of another Christmas staple. Admittedly, Del Boy did eventually become a millionaire but it took him 15 years and an extraordinary slice of luck. Corbyn may also end up being right but if he is, it too will be more down to luck than judgement. The error in his prediction is in starting with a conclusion he wants to be true and working backwards from there.

Not that this is anything new. The original Christmas story should be warning enough of the dangers of reverse-reasoning, if we discount heavenly interventions.

The Sky At Night broadcast a good Christmas Special two years ago, investigating what the real Star of Bethlehem might have been and considering six astronomical options. Three stood out: a triple-conjunction of Jupiter and Saturn, a nova, and a comet. All three have solid evidence behind them (though the nova and the comet are an either-or as the historical reference, from the Chinese, is to a ‘broom star’, which could be either).

If we transport ourselves back a little over 2000 years, a conjunction of Jupiter and Saturn would be astrologically significant: the planets representing the gods of kings and of change. A rare triple-conjunction – where the pair followed each other through the sky for some months, as they did in 6BC – would re-emphasise that significance. But how to apply that knowledge? The appearance of a nova or a comet the next year would provide that answer. The rarity of the conjunction, the astrological nature of the planets involved and the sudden appearance of a guesting celestial body shortly afterwards would be exactly the sort of thing that would send wise men – as those who studied and interpreted the heavens would be – scurrying west.

Would it be enough to find a specific child? Herein lies the problem of having already reached the conclusion. If you follow the stars west and then south, as the heavens might well have guided the magi, you could indeed end up at Bethlehem and, having so arrived, would no doubt be able to find a child born at the relevant time, after local enquiries. The biblical story may well be true in as far as the visit of the wise men is concerned (and if it was, it’s the sort of thing that would have been well-documented as well as being remembered locally – particularly if Herod’s response is as the Bible records – so could well have formed the basis of a Gospel written 70 years later), but just because they set off to find a child and were successful in so doing, that wasn’t necessarily of any greater significance.

Put another way, on a purely rational basis, the logic suggests that extraordinary astronomical events did not appear because of the birth of Christ; instead, Christ was born (or identified) because of extraordinary astronomical events.

We could take this further and suggest that given the paucity of historical evidence for Christ’s early life and the likelihood that if the Bethlehem story is grounded in fact then it would have been remembered thirty or so years later, then it wouldn’t be too difficult for a would-be prophet of about the right age to assume the identity of the child the magi found: why not harness the power of the prophesy? But here we enter controversial and speculative territory.

The relevance to today – or to any time – is simply in the message that it is all too easy to start with a conclusion you want to be true and then rationalise the supporting analysis and logic; for the wish to be father to the thought. Mostly, that just ends up with bad bets. Sometimes though, it changes worlds.

David Herdson