Published in paperback today, though I bought a copy over the weekend, Einstein: His Life and Universe by Walter Isaacson is an excellent biography of the man. He presents a joyous blend of Einstein's life with lovers and friends, employers and events, physics and philosophy. It would serve as a primer in relativity, special and general. It tracks Einstein's reaction to the development of the bomb and his resistance to the Copenhagen interpretation, a position that he held consistently from the early days of his own, seminal contribution to quantum physics.

I have to confess that I didn't start the book from the beginning, but turned to the chapter on 'Einstein's God' first. I'd heard this interview with Isaacson, and felt that here was someone who would present us with the facts without seeking to claim a scalp for either the theist or atheist camp. Sure enough, Einstein is his own man on this question as on all the big ones he tackled in his life. His religious sensibility was based upon his appreciation of nature and that 'behind all the discernible laws and connections, there remains something subtle, intangible and inexplicable. Veneration for this force beyond anything that we can comprehend is my religion.' This leads to perhaps his most famous quote on science and religion: 'science without religion is lame, religion without science is blind.'

He often used the word 'God' and 'Spirit' to refer to this force. In one interview, he explicitly said he was not an atheist, adding 'the problem involved is too vast for our limited minds'; and on another occasion, 'What separates me from most so-called atheists is a feeling of utter humility toward the unattainable secrets of the harmony of the cosmos.' A belief in something larger than himself became a defining part of the humble confidence that tended to characterise his presentation of his famous contributions to science.

However, if that provides succour to the religious, he believed neither in immortality nor in a personal God and, to my mind more challengingly, he was a fierce, causal determinist who would not admit of anything like free will. Determinism and God alike were both implicit in his understanding of science - one implies the other - and he drew on the philosophy of Spinoza to make sense of them, also admiring Spinoza's monism - dealing with the body and soul as one. 'Human beings in their thinking, feeling and acting are not free but are as casually bound as the stars in their motions.' Or drawing on Schopenhauer, he agreed that whilst we are free to do what we will, we are not free to will what we will.

As a result, Einstein suffered from all the expected difficulties faced by a determinist not just in explaining our moral sense but in defending why we should act morally - something that he certainly believed people should do. Roughly, he felt that he was impelled to act as if free will existed, since that makes for a better place to live - though, of course, the argument is incoherent. He admitted as much: 'I know that philosophically a murderer is not responsible for his crime, but I prefer not to take tea with him.'

Incidentally, given the on-going debate here about the significance and beliefs of David Hume, Isaacson's book also tracks Einstein's relationship with the thought of the empiricist philosopher. Roughly, it seems that at the start of his career, he was greatly influenced by Hume. Isaacson says Hume's works were like sacred texts to Einstein. For example, Hume's resistance to anything that could not be experienced, such as Newtonian absolute space and time, prepared conceptual space for Einstein to develop the implications of the principle of relativity. However, at the same time, Einstein's genius also depended upon his willingness to accept some postulates as a priori absolutes, and explore what that implied about the world. Also, Einstein did not seem to take on board Hume's scepticism about causal determinism, since he professed his love of Spinoza, the determinist, from the early days too.

When it came to the development of general relativity, Einstein moved on from the strict empiricism that he identified with Hume (and Mach). For one thing, general relativity depended upon the extension of special relativity not by thought experiment but by mathematics, so a more holistic understanding of the philosophy of science seemed more resonant, as Isaacson puts it. 'No collection of empirical facts, however comprehensive, can ever lead to the formulation of such complicated equations,' Einstein explained. And elsewhere: 'Imagination is more important than knowledge.' Having said that, the great triumph of general relativity was that it made quite extraordinary predictions that were then quite quickly verified by experiment.

In his reaction to quantum physics, he was clearest about his scientific realism. This had two components. First, Bohr had declared that physics was not about what nature is, but only about 'what we can say about nature'. Einstein believed things really existed in nature, independently of our ability to observe or measure them. 'Belief in an external world independent of the perceiving subject is the basis of all natural science.' In this way, he argued that quantum theory must be incomplete, since to his mind, its ethereal nature was profoundly dissatisfying. Second, he objected to the way quantum physics implied abandoning causal determinism, for all the evidence that it does. Hence another famous quote, about God not playing dice - something that, the reference to God admitted, would presumably not bother a Humean.