The second part of M. de Roberty’s work deals in the first place with the conceptions of philosophy, its nature and its end, framed by the three great modern systems. The confusion generally made between philosophy and science is first pointed out, the evil of which arises from the fact that allowance is not made for the progress of scientific knowledge. The author is strongly inclined to favor the idea of the general equivalence of science and philosophy, in the sense that every effect is identified with its cause. But as the effect is always modified with its cause, neither the content not the general conception of philosophy can remain unchangeable. Philosophy becomes thus the co-ordination of the sciences in view of their general and abstract finality—by which is meant simply the last term of an evolution—a conception of the world. In what do the conceptions of philosophy held by the criticist, the positivist, and the evolutionist, differ from that formulated by M. de Roberty? He affirms that they all entertain certain errors of method derived chiefly from ancient metaphysics. The prototype is found in Kant, who says that philosophy is a system of universal acquirements formed of abstract notions, and that it has for its aim the passage of our understanding from sensible to suprasensible knowledge. The latter is the _a priori_, the permanent and verifiable hypothesis, for each of them. It is the transcendental element which all modern philosophy has derived from the past, and which forms the bond of alliance between faith and knowledge. Of the three postulates of Spencer, the universal hypothesis is in the first, an Unknowable Force. The other two belong to psychology, proving that the English evolutionist, like Comte, confounds science with philosophy, which to him, as to his predecessors, is a simple theory of knowledge. Philosophy is a method which conducts to a conception of the world. But, says M. de Roberty, modern philosophies fail in that they deal with hypotheses. Now, although hypothesis is the soul of the special sciences, for philosophy it must always be a purely mental recreation. To render valid the universal hypothesis constructed by philosophers, it would be necessary that the sum of the final truths of science should include the sum of the phenomena which constitute nature.