Necessary character of the forms of errors. Their definite number --Their logical order--Examples of this order in various parts of philosophy--Erring spirit and spirit of search--Immanence of error in truth--Erroneous distinction between possession of and search for truth--Search for truth in the practical sense of preparation for thought; the series of errors--Transfiguration of error into tentative or hypothesis in the search so understood--Distinction between error as error and error as hypothesis--Immanence of the tentative in error itself as error--Individuals and error--Duplicate aspect of errors--Ultimate form of error: the methodological error or hypotheticism. VII THE PHENOMENOLOGY OF ERROR AND THE HISTORY OF PHILOSOPHY Inseparability of phenomenology of error from the philosophical system--The eternal course and recurrence of errors--Returns to anterior philosophies; and their meaning--False idea of a history of philosophy as history of the successive appearance of the categories and of errors in time--Philosophism case in point of this false view, as is the formula concerning the identity of philosophy and history of philosophy--Distinction between this false idea of a history of philosophy, and the books which take it as their title or programme--Exact formula: identity of philosophy and history--History of philosophy and philosophic progress--The truth of all philosophies; and criticism of eclecticism--Researches for authors and precursors of truths; reason for the antinomies which they exhibit. VIII "DE CONSOLATIONE PHILOSOPHIAE" Logic and defence of Philosophy--Utility of Philosophy and the Philosophy of the practical--Consolation of philosophy, as joy of thought and in the true. Impossibility of a pleasure arising from falsity and illusion--Critique of the concept of a sad truth --Examples: Philosophical criticism and the concepts of God and Immortality--Consolatory virtue, pertaining to all spiritual activities--Sorrow and elevation of sorrow. FOURTH PART HISTORICAL RETROSPECT I HISTORY OF LOGIC AND HISTORY OF PHILOSOPHY Reality, Thought and Logic--Relation of these three terms--Inexistence of a general philosophy outside particular philosophic sciences; and, in consequence, of a general History of philosophy outside the histories of particular philosophic sciences--Histories of particular philosophies and literary value of such division--History of Logic in its particular sense--Works dealing with history of Logic. II THEORY OF THE CONCEPT