Faraday, 44, 57, 58, 209 Final explanations, 48 First law of thermodynamics, 126 ff Force, 102 ff Foucault pendulum, 180, 184 Fourth dimension, 74 Future, 222 Gauss, 15, 34 "Go and come" time, 112 Gravitational constant, 91 Haldane, 25 Heat flow, 130 Heisenberg, 222 Hertz, 44 Hoernlé, vi Identity, 91 ff Isolation, 82 Joule, 124 Kelvin, 45, 110 Kinetic theory of gases, 40 Lagrangean equations, 112 Larmor, 149 La Rosa, 164 Length, 9 ff Lewis, 166, 201 Light, 150 ff Local time, 75 Lorentz, 143, 147, 148, 149 Mach, 183 Mass, 102 ff Mathematics, 60 ff Maxwell, 44, 58, 112, 137, 148 Meaningless questions, 28 ff Measurement approximate, 33 Mechanism, 45 Mercury, 105 Michelson, 15, 26 Michelson and Morley, 66 Models, 45, 52 Newton, 4, 110 Operational character of concepts, 5 Operational thinking, 32 Optical space, 67 Ostwald, 109 Penumbra, 34 Perrin, 107 Physical reality, 59 Planck, 69 Poincaré, 48, 115, 116, 190 Pythagoras, 61 Quantum act, 156 Quantum theory, 40, 47, 186 Radiation and temperature, 123 Relative character of knowledge, 25 Relativity, 150 ff Reynolds, 93 Rotational motion, 178 Russell, 205 Schrödinger, 222 Space, 66 Silberstein, 11 Simplicity of nature, 198 Simultaneity, 7, 8 Spring balance, 103 Statistical methods, 115, 117 Stress, 54 Swann, 204 Table top, 106 Tactual space, 67 Temperature, 118 ff Thermodynamics, 117 Thing traveling, 101, 152, 157, 164 Time, 69 Tolman, 201 Truth, 78 Turbulent motion, 120, 124 Velocity, 97 ff, 213 ff Velocity of light, 100 Whitehead, 167 Wilson, 224