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Sometimes two theories will predict exactly the same observable consequences. Here's a simple illustration. Suppose that, at 7 a.m. each morning I take a walk along the beach. And each morning I discover another set of footprints already there — shoe size 10. I might come up with two theories to account for what I have observed:

Theory 1. Each morning a person with size 10 feet walks down the beach before 7 a.m.

Theory 2. Each morning a person with size 10 feet — and with an invisible, intangible fairy sitting on their head — walks down the beach before 7 a.m.

These two theories both explain what I have observed. Not only that, but they make the exact same predictions about what I would observe were I to rise earlier and track down the mysterious morning footprint-leaver. Which theory is best?

Clearly, theory 1 is preferable to theory 2. It is more likely to be true. But why? Because it is simpler. Why complicate matters with the fairy?

Here's another example. Why does the universe exist? Why, indeed, is there anything at all? Here are two explanations:

Four Books of Theory 1. The universe was created by a god. the Sentences

Sum of Logic Theory 2. The universe was created by a god, who created a second god, who in turn created the universe.

Quodlibeta

Septem These two theories predict the very same observable consequences — the existence of the universe. Yet the first

Biography BORN c.1288, IN OCKHAM, SURREY, ENGLAND. William of S4 w Ockham (or Occam) is one of the most important medieval Le

thinkers. Little is known of his early life — he studied theology at Oxford from 1309 to 1321, pa but never completed his Masters degree. He was a Franciscan, and became deeply involved in a dispute between the papacy and the Franciscans. In 1327 the Franciscan Minister General (administrative head of the order), Michael of Cesena, was summoned to the papal

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court (then located in Avignon, not Rome) on charges of heresy, having argued that Jesus and at

the Apostles lived in poverty (as did many Franciscans but not, of course, the Pope and his Tr

retinue). Ockham was asked to investigate the matter, and did so, concluding that the de Minister General was correct and that it was in fact the Pope who was guilty of heresy for continuing to assert what he had been shown to be false. Ockham, Cesena and other Franciscans fled Avignon into exile. They found refuge under the protection of Ludwig of T1 Bavaria, where Ockham continued his career and remained for the rest of his life. Ockham was excommunicated in 1328 for leaving Avignon without permission.

Died c.1348, in Munich, Bavaria, now part of Germany Ty ev

50 WILLIAM OF OCKHAM

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A popular principle OCKHAM HIMSELF DID NOT devise Ockham's razor. The principle was

already well known in medieval times. Aquinas (see page 43), for example, presents a version of it:

If a thing can be done adequately by means of one, it is superfluous to do it by means of several; for we observe that nature does not employ two instruments where one suffices.

The reason the principle has become associated with Ockham is that Ockham made particularly frequent use of it.

The principle can be formulated in a number of different ways. The version usually associated with Ockham is:

Entities should not be multiplied beyond necessity.

However, these words do not, in fact, appear to be Ockham's. Ockham himself said:

Plurality ought never be posed without necessity.

Note that although philosophers regularly apply Ockham's razor, it is, of course, also popular with scientists. Isaac Newton says:

We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.

Both hypotheses account for everything you have experienced up to now. They also make the same predictions concerning what you might expect to observe. In which

case, concludes the sceptic, they must be equally reasonable.

But of course, now armed with Ockham's razor, we might well question that conclusion. Are the two hypotheses equally reasonable? What if one is simpler than

the other? We might suggest, for example, that the first theory is simpler. In effect, the

second theory posits two worlds. It posits a real world - containing an evil demon - in which a second illusory world is generated. As the first theory invokes just one

world, it is therefore simpler. The trouble is, we might also argue like this: the first theory invokes very many

entities - tables and chairs, mountains and seas, and so on - whereas the second invokes just two: your mind and that of the demon. Therefore the second theory is simpler.

You can now see one of the difficulties we can face when applying Ockham's

razor. Even if we interpret the razor as applying to entities, what is to count as an

'entity'? Ockham's razor is undoubtedly a valuable philosophical and scientific tool. However, it is not always obvious precisely how it should be applied.

52 WILLIAM OF OCKHAM