His attempt to reduce the principle of causality to that of identity did not succeed very well, and from the nature of the case this was to be expected. As Fouillée well points out in his criticism of Taine, both in _La Liberté et le Déterminisme_ and the concluding pages of his earlier work on Plato,[15] the notion of difference and heterogeneity which arises in the action of cause and effect can never be reducible to a mere identity, for the notion of identity has nothing in common with that of difference. Differences cannot be ignored; variety and change are undeniable facts of experience. Fouillée here touches the weak spot of Taine’s doctrine. In spite of a seemingly great power of criticism there is an underlying dogmatism in his work, and the chief of those dogmas, which he does not submit to criticism, is the assertion of the universal necessity of all things. To this postulate he gives a false air of objectivity. He avoids stating why we do objectify causality, and he diverts discussion from the position that this postulate may itself be subjective. [15] Vol. 4. The particular bearing of Taine’s psychology upon the general problem of knowledge is interesting. He defines perception in _L’Intelligence_ as _une hallucination vraie_. His doctrine of the “double aspect,” physical and mental, recalls to mind the Modes of Spinoza. In his attitude to the difficult problem of movement and thought he rests in the dualism of Spinoza, fluctuating and not enunciating his doctrine clearly. The primacy of movement to thought he abandoned as too mechanical a doctrine, and regarded the type of existence as mental in character. Taine thus passes from the materialism of Hobbes to the idealism of Leibnitz. “The physical world is reducible to a system of signs, and no more is needed for its construction and conception than the materials of the moral world.” When we feel ourselves constrained to admit the necessity of certain truths, if we are inclined to regard this as due to the character of our minds themselves (_notre structure mentale_), as Kant maintained, Taine reminds us that we must admit that our mind adapts itself to its environment. He here adopts the view of Spencer, a thinker who seems to have had far more influence upon the Continent than in his own country. Although Taine thus reposes his epistemology upon this basis, he does not answer the question which the Kantian can still put to him—namely, “How do we know the structure of things?” He is unable to escape from the difficulty of admitting either that it is from experience, an admission which his anti-empirical attitude forbids him to make (and which would damage his dogma of universal logical necessity), or that our knowledge is obtained by analysing our own thoughts, in which case he leaves us in a vicious circle of pure subjectivity from which there is no means of escape.