At an early period of Mr. Mill's philosophical life, Hartley's work had taken a strong hold of his mind; and in the maturity of his powers he formed and executed the purpose of following up Hartley's leading thought, and completing what that thinker had begun. The result was the present work, which is not only an immense advance on Hartley's in the qualities which facilitate the access of recondite {xviii} thoughts to minds to which they are new, but attains an elevation far beyond Hartley's in the thoughts themselves. Compared with it, Hartley's is little more than a sketch, though an eminently suggestive one: often rather showing where to seek for the explanation of the more complex mental phenomena, than actually explaining them. The present treatise makes clear, much that Hartley left obscure: it possesses the great secret for clearness, though a secret commonly neglected--it bestows an extra amount of explanation and exemplification on the most elementary parts. It analyses many important mental phenomena which Hartley passed over, and analyses more completely and satisfactorily most of those of which he commenced the analysis. In particular, the author was the first who fully understood and expounded (though the germs of this as of all the rest of the theory are in Hartley) the remarkable case of Inseparable Association: and inasmuch as many of the more difficult analyses of the mental phenomena can only be performed by the aid of that doctrine, much had been left for him to analyse.