Kant's nutshell argument for idealism

The significance or vacuity of the statement, “Everything has just doubled in size,” attracted considerable attention last century from scientists and philosophers. Presenting his conventionalism in geometry, Poincaré insisted on the emptiness of a hypothesis that all objects have doubled in size overnight. Such expansion could have meaning, he argued, “only for those who reason as if space were absolute … it would be better to say that space being relative, nothing at all has happened.” The logical empiricists concurred, viewing the universal doubling hypothesis as illustrating the intrinsic metrical amorphousness of continuous manifolds. It is striking, therefore, to find Kant invoking a universal contraction in space and time to support his famous doctrine of transcendental idealism. In one of several completely neglected passages, he writes: “The proof that the things in space and time are mere appearances can also be grounded on the fact that the whole world could be contained in a nutshell and the entirety of elapsed time in a second without the least difference being met with.” Kant's “also” may suggest an idealist argument distinct from any proposed in published works. Here I ask: What is the meaning of Kant's Nutshell Argument for Idealism?