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“The most important result of our reflections is …that precisely the apparently simplest mechanical theorems are of a very complicated nature; that they are founded on incomplete experiences, even on experiences that never can be fully completed; that in view of the tolerable stability of our environment they are, in fact, practically safeguarded to serve as the foundation of mathematical deduction; but that they by ho means themselves can be regarded as mathematically established truths, but only as theorems that not only admit of constant control by experience but actually require it.”