“In his maturity, Einstein more firmly believed that there was an objective ‘reality’ that existed whether or not we could observe it. The belief in an external world independent of the person observing it, he repeatedly said, was the basis of all science.

In addition, Einstein resisted quantum mechanics because it abandoned strict causality and instead defined reality in terms of indeterminacy, uncertainty, and probability. A true disciple of Hume would not have been troubled by this. There is no real reason—other than either a metaphysical faith or a habit ingrained in the mind—to believe that nature must operate with absolute certainty. It is just as reasonable, though perhaps less satisfying, to believe that some things simply happen by chance. Certainly, there was mounting evidence that on the subatomic level this was the case.

But for Einstein, this simply did not smell true. The ultimate goal of physics, he repeatedly said, was to discover the laws that strictly determine causes and effects. ‘I am very, very reluctant to give up complete causality,’ he told Max Born.

His faith in determinism and causality reflected that of his favorite religious philosopher, Baruch Spinoza. ‘He was utterly convinced,’ Einstein wrote of Spinoza, ‘of the causal dependence of all phenomena, at a time when the success of efforts to achieve a knowledge of the causal relationship of natural phenomena was still quite modest.’ It was a sentence that Einstein could have written about himself, emphasizing the temporariness implied by the word ‘still,’ after the advent of quantum mechanics.

Like Spinoza, Einstein did not believe in a personal God who interacted with man. But they both believed that a divine design was reflected in the elegant laws that governed the way the universe worked.

This was not merely some expression of faith. It was a principle that Einstein elevated (as he had the relativity principle) to the level of a postulate, one that guided him in his work. ‘When I am judging a theory,’ he told his friend Banesh Hoffmann, ‘I ask myself whether, if I were God, I would have arranged the world in such a way.’

When he posed that question, there was one possibility that he simply could not believe: that the good Lord would have created beautiful and subtle rules that determined most of what happened in the universe, while leaving a few things completely to chance. It felt wrong. ‘If the Lord had wanted to do that, he would have done it thoroughly, and not kept to a pattern . . . He would have gone the whole hog. In that case, we wouldn’t have to look for laws at all.’

This led to one of Einstein’s most famous quotes, written to Max Born, the friend and physicist who would spar with him over three decades on this topic. ‘Quantum mechanics is certainly imposing,’ Einstein said. ‘But an inner voice tells me that it is not yet the real thing. The theory says a lot, but it does not really bring us any closer to the secrets of the Old One. I, at any rate, am convinced that He does not play dice.’

Thus it was that Einstein ended up deciding that quantum mechanics, though it may not be wrong, was at least incomplete. There must be a fuller explanation of how the universe operates, one that would incorporate both relativity theory and quantum mechanics. In doing so, it would not leave things to chance.”

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From Walter Isaacson’s biography Einstein: His Life and Universe.

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