One of the problems that motivated Kant (1787) when he wrote The Critique of Pure Reason was that the epistemological and ontological claims of both rationalism and empiricism presupposed the notion of 'the world as a whole'. This, for him, was not an object of experience, and hence was something about which we could not have legitimate knowledge. In the final chapter of their book The Quest for Unity, Klein and Lachieze-Rey ask what lies behind pronouncements about an impending 'theory of everything' and question whether such a hope is, to use their words, 'a bit brash'. They claim, as Kant did over 200 years ago, that no experiment has been conducted, or can ever be conducted, on the world as a whole. Hence, it is highly unlikely that the answers required for a theory of everything will come from experiments. Consequently, the theory may be condemned to remain forever pure conjecture (p. 116).
Should the reader conclude from such remarks that Klein and Lachieze-Rey are sceptical of unity? Not entirely. Throughout the book they attempt to occupy a middle ground between praise for the unifying achievements of physics and caution toward how those achievements ought to be interpreted. They warn against metaphysical contemplation that inspires bold hypotheses but at the same time can lure scientific thinking into an arrogant overconfidence that it is within reach of its destiny: 'Only if it renounces pretensions to be a perfect reflection of reality will physics continue to thrive ' (p. 131). Yet, they claim that to give up searching for the unity underlying diverse phenomena would be tantamount to giving up on physics itself. Although the search for unifications remains the guiding principle of physics, $ I would like to thank Jeremy Butterfield for helpful editorial comments.