These characteristics, combined perhaps with a greater share of originality, appear in the _System of Logic_ as well as in the Utilitarian treatises. Its merit is proved by the fact that through many years of adverse criticism it has maintained its ground at the universities as one of the most useful books on the subject. The freshest section is that which is devoted to Induction. The _Examination of Hamilton_ shows Mill to have possessed the gift of acute and powerful criticism of philosophy. He may not have succeeded in establishing his own position, but he certainly damaged very seriously the rival system of Hamilton. Mill's _Political Economy_ is, like his general philosophy, lucid, full and thorough. Though cautious here, as always, in the admission of new principles, Mill made considerable contributions to economics. The theory of international exchanges is almost wholly his, and many particular turns and details of economic doctrine are due to him. In a still greater number of cases he has been, not the originator, but the best exponent of economic theory. The caution and judiciousness of his reasoning were qualities peculiarly valuable in this sphere; and where the views of 'orthodox' political economy are accepted at all, Mill's opinions are treated with respect. The time when Mill's authority was at its height was also the time when political economy was held in greatest honour as a science. The writers on it were numerous; and though, with the exception of Mill, they were not individually very distinguished, their collective work was important. They developed the doctrines of Adam Smith and Ricardo and Mill; while the speculations of Malthus acquired through Darwin a new importance, until a reaction, brought about more by sentiment than reason, led many to the conviction, or the faith, that they could not possibly be sound. The doctrine of _laissez faire_, so influential on government during the third quarter of the century, was the work partly of the economists and partly of the practical politicians of the Manchester school. It was never followed out logically, and before the close of the period there were signs of a movement which has since led to an opposite excess. Of the men who did this work Nassau W. Senior (1790-1864), in the earlier part of the period, and J. E. Cairnes (1823-1875) in the later deserve individual mention. The former was a great upholder of the deductive theory of political economy. The latter, in his treatise on _The Slave Power_ (1862), produced one of the most noteworthy special studies in economics, and also one of the most powerful arguments in favour of the action of the Northern States of America.