To the casual thinker it may appear that the arguments of Dr. Mivart are somewhat antiquated as against the strongholds of modern error; but the fact additionally illustrates the slenderness of the resources with which error comes equipped to the fray, since, whenever there is question of first principles, truth can with the same weapons always assail the vulnerable point in the enemy’s armor. It is true that in point of detail the ground of conflict has shifted, and that those who once successfully opposed the errors of Voltaire, Diderot, or Volney, should they suddenly appear on the scene now, would have to count themselves out of the fight; but with respect to principles and ultimate expressions, we find the Agnostics of to-day ranging themselves side by side with the Gnostics and Manicheans of old. So we believe that Dr. Mivart has done well, before approaching the details of the controversy, to knock the underpinning from the whole superstructure of modern error by exposing the falsity of its principles. At least the procedure is more philosophical and more satisfactory to the logical mind. In his second chapter, entitled “First Truths,” Dr. Mivart lays down the following proposition: “Knowledge must be based on the study of mental facts and on undemonstrable truths which declare their own absolute certainty and are seen by the mind to be positively and necessarily true.” This proposition finds its counterpart in every text-book of scholastic philosophy from Bouvier to Liberatore and Ton Giorgi, so that there is no need to follow the learned author through his very excellent series of proofs in support of it. The main points of interest in the chapter are his arraignment of Herbert Spencer’s faulty basis of certainty, and the disproof of Mr. Lewes’ theory of reasoning. Mr. Spencer says (_Psychology_, vol. ii. p. 450): “A discussion in consciousness proves to be simply a trial of strength between different connections in consciousness――a systematized struggle serving to determine which are the least coherent states of consciousness. And the result of the struggle is that the least coherent states of consciousness separate, while the most coherent remain together; forming a proposition of which the predicate persists in the mind along with its subject.… If there are any indissoluble connections, he is compelled to accept them. If certain states of consciousness absolutely cohere in certain ways, he is obliged to think them in those ways.… Here, then, the inquirer comes down to an ultimate uniformity――a universal law of thinking.”