Here’s a satisfying little allegory from the ‘Doctrine of Method’. Kant tells us that we must give up our ambitious designs for human knowledge, turning away from rationalism and its metaphysics which wanted to build a “tower reaching to the heavens”. Instead, we must remain content with something altogether more modest, merely a “dwelling house” which is “just roomy enough for our tasks on the plain of experience”.
If we look upon the sum of all knowledge of pure speculative reason as a building for which we have at least the idea within ourselves, it can be said that in the ‘Transcendental Doctrine of Elements’ we have made an estimate of the materials, and have determined for what sort, height and strength of building they will suffice. Indeed, it turned out that although we had in mind a tower that would reach the heavens, yet the stock of materials was only enough for a dwelling house — just roomy enough for our tasks on the plain of experience and just high enough for us to look across the plain. The bold undertaking had come to nothing through a lack of materials, quite apart from the babel of tongues that unavoidably set workers against one another about the plan and scattered them across the earth, each to build separately following his own design. Our problem is not just to do with materials, but even more to do with the plan. Since we have been warned not to risk everything on a favourite but senseless project, which could perhaps exceed our whole means, yet cannot well refrain from building a secure home, we have to plan our building with the supplies that have been given and also to suit our needs.
CPR A707/B735 trans. O’Neill