This return to the philosophical tradition, which between the end of the last and the beginning of this century, was not limited to Croce and to Italy only, was accompanied and indirectly favoured by the researches of pure scientists on the method of exact and natural sciences. The economic theory of the scientific concept, such as it appears especially in the works of Mach and Avenarius, and to an understanding of which Croce had been prepared by his own studies on Marxism, was probably the most efficient instrument in destroying from within the pseudo-scientific constructions of positivism. The scientists themselves, by defining the limits of scientific thought, proved the impossibility of building a philosophy which should be at the same time a synthesis of all particular sciences and a system of reality. The conclusions of this new scientific methodology are on the whole accepted by Croce, and the fact that they naturally fall into their proper place in his logic is the most valid justification of his method, to which the distinction between the concept of philosophy and the concepts of the sciences is essential. We need not point to the object of logic, or concept, as we did in a former chapter to the object of æsthetics, or intuition. The writing of this book implies a belief in its existence, and we could take practically any page of it as an example of what we mean by concept, or logical knowledge. We shall not therefore pause to confute logical scepticism, except by repeating the old argument that it is impossible to deny the existence of the concept except through the formulation of a concept. Such affirmations as that there is no other knowledge than the æsthetic one, or the one which is given by the ineffable intuition of the mystic, or by practical fictions, are in their turn neither æsthetic knowledge nor mystical intuitions, nor practical fictions, but affirmations, however contradictory in themselves, of a universal value and of an absolute character, that is, concepts. Through them, it is possible immediately to distinguish the logical form of knowledge, as represented by such affirmations, from the æsthetic or representative one, from the sentimental or practical state of mind of the mystic, and from those concepts which are mere empirical fictions. It is evident, in this last instance, that the theory of the fiction cannot be a new fiction, but must belong to an activity of a different kind, the logical activity, whose value is truth.