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Review of 'The Grand Design' by Stephen Hawking and Leonard Mlodinow - part 4: Model-dependent Realism

The Grand Design by Stephen Hawking and Leonard Mlodinow

In this last part of my review of 'The Grand Design,' by Stephen Hawking and Leonard Mlodinow, I want to talk about what they call 'Model-Dependent Realism.' This is quite a technical issue, so why bother with it? The answer is that they use it to fudge the question of truth.

The idea of Model-Dependent Realism is that we build mental pictures, or models, of how the world works, and we interpret the world through those models. We don’t have direct access to the world as it actually is, only to the world as it is filtered through our model. So they say,

‘There is no picture- or theory-independent concept of reality’ (p. 42).

So far, so good. Most thinking people would probably agree with this. There is nothing very new about it. Back in the 18th century, Immanuel Kant said that we cannot know the noumenal world – the world as it actually is, only the phenomenal world – the world as it appears to our senses and understanding.

The trouble is not just that Hawking and Mlodinow are dismissive of philosophy; they are philosophically naïve. And in their use of model-dependent realism, they are inconsistent: they lurch off in two contradictory directions:

1. Truth is what you make it

Sometimes they lurch in the direction of saying that truth is whatever you make it (and therefore, perhaps what is true for you does not need to be true for me too.) So they say

‘Your reality depends on the model you employ.’ (page 175)


‘There is no model-independent test of reality. It follows that a well-constructed model creates a reality of its own.’ (page 172).

The trouble with this lies in the phrase ‘It follows that…’ - because it doesn’t really follow at all.

The fact that I have no access to reality except through my model of the world does not imply that my model somehow creates the reality. To say that one follows from the other is simply a failure to think logically.

An example may help: Ptolemy believed that the Earth was at the centre of the universe, and the planets, the Sun and the Moon went around the Earth. Copernicus believed that the Earth and the other planets all go round the Sun. Either model can be used to predict the positions of the planets. In fact, the (highly-polished) system of Ptolemy did a better job of prediction than the (rough round the edges, slightly inaccurate) system of Copernicus.

According to Hawking and Mlodinow, these two systems are equivalent. But they aren’t equivalent – really aren’t equivalent. No-one today, unless they’re barking mad, thinks that the Sun goes round the Earth, or that it is just a matter of your point of view. We do actually, as a matter of fact, truth, and reality, think that the Earth goes around the sun.

We don’t think that our model creates the reality, however well-constructed it may be.

This is because there is something outside the model that we can check it against. Our model can be falsified. Because of this, some models really are better than others. We may never have access to reality that is free of our models and theories, but we can test that some models and theories are better than others.

2. Critical realism

As soon as you say this, you have rejected the idea of truth-as-what-you-make-it, in favour of something that philosophers call ‘critical realism.’

Critical realism says that we may not be able to get to grips with reality itself, with absolute truth – but we can get closer to it. Some models are genuinely better than others, and we can test how good a model is by the predictions it makes. So, for example, we believe that Einstein’s Theory of General Relativity is a better model of reality than Newton’s theory of gravity (even though for many practical purposes Newton’s theory is good enough).

Hawking and Mlodinow sometimes lurch into critical realism, without realising what they have done. For example, they say:

‘But the division of natural forces into four classes is probably artificial and a consequence of our lack of understanding.’ (page 109)

This is a view that makes perfect sense within Critical Realism, but just what does it mean within Model-Dependent Realism? If my model of the world creates the reality, it is meaningless to say that something is artificial, or a result of lack of understanding.

Most practising scientists are probably critical realists. Model-Dependent Realism is not the same thing as critical realism. So the philosophical position taken up by Hawking and Mlodinow in ‘The Grand Design’ is probably not the position taken by most practising scientists.

In laying claim to Model-Dependent Realism, Hawking and Mlodinow lurch backwards and forwards between a kind of postmodern relativism, in which truth is what you make it, and Critical Realism, in which the truth is out there, and we may approach it more and more closely, without ever actually reaching it. Once again, they are showing their lack of philosophical savvy. At the same time, they are confusing the issue by jumping between different views of truth and knowledge.

Go to the previous part of this review: M-theory and Multiverses.

Order 'The Grand Design' from Amazon USA (hardback)

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Review article by Professor John Lennox.

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