Ultimately he settled down to metaphysical and ethical inquiries, and devoted to these the last thirteen years of his life. During his undergraduate years the two intellectual forces most powerful at Oxford had been the writings of J. H. Newman in the religious sphere, though their influence was already past its meridian, and the writings of John Stuart Mill in the sphere of logic and philosophy. By neither of these, save in the way of antagonism, had Green been influenced. He heartily hated all the Utilitarian school, and had an especial scorn for Buckle, who, now almost forgotten, enjoyed in those days, as being supposed to be a philosophic historian, a brief term of popularity. Green had been led by Carlyle to the Germans, and his philosophic thinking was determined chiefly by Kant and Hegel, more perhaps by the former than by the latter, for it was always upon ethical rather than upon purely metaphysical problems that his mind was bent. His religious vein and his hold upon practical life made him more interested in morals than in abstract speculation. Thus he became the leader in Oxford of a new philosophic school which looked to Kant as its master, and which for a time, partly perhaps because it effectively attacked the school of Mill, received the adhesion of some among the most thoughtful of the younger High Churchmen. Like Kant, he set himself to answer David Hume, and the essay prefixed to his edition of Hume's _Treatise on Human Nature_. along with his _Prolegomena to Ethics_, are the only books in which his doctrines have been given to the world, for he did not live to write the more systematic exposition he had planned. These two essays are hard reading, for his philosophical style was usually technical, and sometimes verged on obscurity. But when he wrote on less abstruse matters he was intelligible as well as weighty, full of thought, and with an occasional underglow of restrained eloquence. The force of character and convictions makes itself felt through the language.
His mind, though constructive, was not, having regard to its general power, either fertile or versatile. Like most of those who prefer solitary musings to the commerce of men, he had little facility, and found it hard to express his thoughts in any other words than those into which his musings had first flowed. Thus even his oral teaching was not easy to follow. An anecdote was current how when one day he had been explaining to a small class his theory of the origin of our ideas, the class listened in rapt attention to his forcible rhetoric, admiring each sentence as it fell, and thinking that all their difficulties were being removed. When he ended they expressed their gratitude for the pleasure he had given them, and were quitting the room, when one, halting at the door, said timidly, "But, Mr. Green, what did you say was really the origin of our ideas?" However, whether they were or were not capable of assimilating his doctrines, his pupils all joined in their respect for him. They felt the loftiness of his character, they recognised the fervour of his belief. He was the most powerful ethical influence, and perhaps also the most stimulative intellectual influence, that in those years played upon the minds of the ablest youth of the University. But it was a singular fact, which those who have never lived in Oxford or Cambridge may find it hard to understand, that when he rose from the post of a college tutor to that of a University professor, his influence declined, not that his powers or his earnestness waned, but because as a professor he had fewer auditors and less personal relation with them than he had commanded as a college teacher. Such is the working of the collegiate system in Oxford, curiously unfortunate when it deprives the ablest men, as they rise naturally to the highest positions, of the opportunities for usefulness they had previously enjoyed.