 Observation and experimentation are no more central to contemporary scientific practice than is quantification. Contemporary physical theory, which Spinoza is said to have anticipated, is thoroughly quantitative in character. Theories are expressed in quantitative terms; giving an explanation of a phenomenon is typically a matter of generating a mathematically formulated law that covers it. Unifying theories is a matter of showing how the mathematical formulae that comprise them can be derived from one another, how the phenomena they concern can be commensurated. Indeed, observation itself is irrelevant to modern physical theory unless it is expressed in quantitative terms, since the prediction yielded by physical theories are predictions of what the observed measures of things will be. This, in turn, requires that the phenomena observed must be measurable.