Einstein has recorded that in the years before he published his paper on Special Relativity in 1905, he was fortified in his speculations by the sceptical remarks of David Hume in his Treatise on Human Nature, where Hume stated that the idea of a perfect connection of imperfect real measures ‘is a mere fiction of the mind, as well as useless and incomprehensible.’ Hume extended his scrutiny from considerations of spatial measure to notions of the duration of time: ‘… where though it is evident that we have no exact method of determining the proportion of parts, not even so exact as in extension, yet the various corrections of our measures, and their different degrees of exactness, have given us an obscure and implicit notion of a perfect and exact equality.’ And in Ernst Mach, Einstein found a warning against the use of concepts disconnected from their experiential grounding.
Einstein was, I suppose, throwing off the immense prestige of Newton, who had insisted on the priority of space and time in experience, and hence their absolute inviolability. In these reflections about the origins of his theory of Special Relativity, it is perhaps surprising that Einstein does not mention Leibniz, but that is perhaps an historical accident, and relates to the activities of the reading group of which he was a member in Bern during the first years of the twentieth century. Leibniz, after all, was of the opinion that space was an order of co-existing phenomena, and time an order of successive phenomena, and emphasised the relational aspect rather than fixed distances and durations.
Kant’s mature views on space and time are set out in the Transcendental Aesthetic of the Critique of Pure Reason. With extraordinary originality he preserves Newton’s views on the absoluteness of space and time, by regarding them as the forms of outer and inner sense, and therefore pre-conditions of any experience at all. We now know that this won’t do. Einstein’s theoretical considerations predicted that space and time are not absolute, and mirabile dictu, quantitative observational confirmation of his predictions has been found. Unsurprisingly Einstein was not a neo-Kantian. For example he wrote in his essay The Problem of Space: ‘ …concepts have reference to sensible experience, but they are never, in a logical sense, deducible from them. For this reason I have never been able to understand the quest of the a priori, in the Kantian sense. In any ontological question, the only possible procedure is to seek out these characteristics in the complex of sense experience to which these concepts refer.’ Bertrand Russell shared Einstein’s view that Kant was overrated for similar reasons. Personally I think they missed an important insight in Kant, and that there are straws in the wind which suggest the problem of consciousness, so central to him and addressed above all in his profound account of the nature of self, will haunt the philosophy of science in the decades to come.
Let me add a little more about Einstein. If he sounds at times like a realist (a description he rejected as describing him in contrast to other ‘isms) or with an intellectual bias towards empiricism, he had opinions which were not wholly consistent. For instance he said, ‘So far as the theories of mathematics are about reality they are not certain, so far as they are certain they are not about reality’. And yet in the Herbert Spencer Lecture of 1933 he said, ‘But the creative principle resides in mathematics. In a certain sense, therefore, I hold that pure thought can grasp reality, as the ancients dreamed.’ In his address in Berlin on the occasion of Max Planck’s sixtieth birthday in 1918, he said, ‘Nobody who has gone really deeply into the matter will deny that in practice the world of phenomena uniquely determines the theoretical system, in spite of the fact that there is no logical bridge between phenomena and their theoretical principles; this is what Leibniz described so happily as a ‘pre-established harmony.’
Planck and Einstein became close friends in Berlin, so it is perhaps appropriate to conclude with two quotations by Planck which have a Kantian feel. ‘I regard consciousness as fundamental. I regard matter as derivative from consciousness. Everything that we talk about, everything that we regard as existing presupposes consciousness.’ (1931). And ‘Science cannot solve the ultimate mystery of nature. And that is because, in the last analysis, we ourselves are part of nature, and therefore part of the mystery we are trying to solve’. (1932).