The last essays of the book on _Materialismo Storico_ are two letters to Professor Pareto "On the Economic Principle," written in 1900; but with these we reach a time when Croce's thought was already organizing itself in the system of the philosophy of mind. In them we find a sketch of the system in the form in which it appeared in the first edition of the _Estetica_: that is, we already decidedly enter into the maturer phase of Croce's thought. We must here pause on the threshold, and looking back on the years of Croce's special interest in economic problems, sum up the new elements that the study of these problems adds to his intellectual physiognomy: a more deliberately anti-metaphysical attitude, a growing consciousness of the complexity of history and of the concreteness of moral life, a realization of the function of the economic activity, a progress in the analysis of scientific concepts, and therefore in the foundations of his logic--but, most important of all, a continued practice of philosophical thought under the shape of historical methodology. Apart from their interest as documents of the growth of his philosophy, Croce's studies have also a place in the history of social and economic thought, side by side with those of Labriola and of Georges Sorel, as a significant episode in that Latin crisis of Marxism, the ultimate outcome of which are the theories of French and Italian Syndicalism. [Footnote 1: _Contributo,_ p. 36-37.] PART SECOND THE PHILOSOPHY OF MIND (1900-1910) I. THE GROWTH OF THE SYSTEM The unity of thought--The writing of the Estetica--The method of philosophy--A philosophy of mind--The _Filosofia della Spirito_ and the _Critica_--Other activities.