The middle of the 1830s may be viewed as a crucial moment in J.S. Mill's intellectual development also with regard to his interests in psychological matters. Under the influence of romantic culture and owing to his involvement in the debate about the nature of poetry, the young Mill was seriously leaded to reflect on the individual mental differences in a way not so usual in the associationist tradition in which he was brought up. At that time he believed he could find a solution in ascribing these phenomena to different degrees of nervous susceptibility apt to giving rise, through the operations of the laws of mind, to different mental characters. Ever since then he steered his approach to psychology towards a new version of scientific associationism, seriously concerned with the physical basis of mental phenomena but severely critic at the same time of any form of reductionism such as Auguste Comte's adoption of phrenological physiology.