Murray Rothbard
Libertarian Man
Rothbard in the 1970s
Rothbard was an advocate and practitioner of the Austrian School tradition of his teacher Ludwig von Mises.
Like Mises, Rothbard rejected the application of the scientific method to economics and dismissed econometrics, empirical and statistical analysis and other tools of mainstream social science as outside the field (economic history might use those tools, but not Economics proper).
He instead embraced praxeology, the strictly a priori methodology of Mises.
Praxeology conceives of economic laws as akin to geometric or mathematical axioms: fixed, unchanging, objective and discernible through logical reasoning.
Read more in Wikipedia.