 The previous subsection discussed the axiomatization of empirical structures, a line of inquiry that dates back to the early days of measurement theory. A complementary line of inquiry within measurement theory concerns the classification of measurement scales. The psychophysicist S.S. Stevens (1946, 1951) distinguished among four types of scales: nominal, ordinal, interval and ratio. Nominal scales represent objects as belonging to classes that have no particular order, e.g., male and female. Ordinal scales represent order but no further algebraic structure. For example, the Mohs scale of mineral hardness represents minerals with numbers ranging from 1 (softest) to 10 (hardest), but there is no empirical significance to equality among intervals or ratios of those  numbers.[5]  Celsius and Fahrenheit are examples of interval scales: they represent equality or inequality among intervals of temperature, but not ratios of temperature, because their zero points are arbitrary. The Kelvin scale, by contrast, is a ratio scale, as are the familiar scales representing mass in kilograms, length in meters and duration in seconds. Stevens later refined this classification and distinguished between linear and logarithmic interval scales (1959: 31–34) and between ratio scales with and without a natural unit (1959: 34). Ratio scales with a natural unit, such as those used for counting discrete objects and for representing probabilities, were named “absolute” scales.