Nontechnical essays on hypothesis in physical theory, concept of number, magnitude, force, intuition vs. logic, more. Chapters include "On the Nature of Mathematical Reasoning," "Mathematical Magnitude and Experiment," "Non-Euclidean Geometries," "Space and Geometry," "Experiment and Geometry," The Classical Mechanics," "Energy and Thermo-Dynamics," "Hypotheses in Physics," and "The Calculus of Probabilities."