P. J. Proudhon 
The Philosophy of Misery [EPUB ebook] 

I affirm the REALITY of an economic science.
This proposition, which few economists now dare to question, is the boldest, perhaps, that a philosopher ever maintained; and the inquiries to follow will prove, I hope, that its demonstration will one day be deemed the greatest effort of the human mind.
I affirm, on the other hand, the ABSOLUTE CERTAINTY as well as the PROGRESSIVE nature of economic science, of all the sciences in my opinion the most comprehensive, the purest, the best supported by facts: a new proposition, which alters this science into logic or metaphysics in concreto, and radically changes the basis of ancient philosophy. In other words, economic science is to me the objective form and realization of metaphysics; it is metaphysics in action, metaphysics projected on the vanishing plane of time; and whoever studies the laws of labor and exchange is truly and specially a metaphysician.