The Methods of Observation of Quantity in general, are 'Numeration', which is precise by the nature of Number; the 'Measurement of Space' and 'of Time', which are easily made precise; the 'Conversion of Space and Time', by which each aids the measurement of the other. The Idea of Space becomes more clear by studying 'Geometry'; the Idea of Force, by studying 'Mechanics'; the Ideas of Likeness, of Kind, of subordination of Classes, by studying 'Natural History'.