The fact is that the history of Greek thought had shown conclusively the absolute futility of any efforts to arrive at a certain proof of the existence of God by purely rational methods. The attempts of each school to attain such certainty were repudiated by their successors, and even by their contemporaries; and the later trials--which the religious instincts and aspirations of men would not permit them to forego, even when they were sceptical of obtaining any valid and positive results--frequently became, instead of a sincere seeking after God, mere practice in the art of Rhetoric. And not only was it true that no one of the forms of the theistic argument brought conviction to any other mind than that of the man who regarded it with the partial enthusiasm of an originator or formulator, but even such an one was led to only the most vague and indefinite results. We have already seen how even the best theology of the Greeks led to nothing but a sort of organized or unified polytheism. A vague, fanciful first cause of physical phenomena, a general idea, abstracted out of all content, so as to leave no meaning for the human mind--whatever the imagination might make of it--a mechanical, magnetic force, to which all motion might conveniently be referred; a deified principle of order--and these held in conjunction with the popular polytheism, and impregnated with the national pantheistic conceptions--was all that Greek philosophy could offer to the higher religious aspirations of the educated man. The opinion of the Greek mind itself as to the character of the knowledge of God, to which the thought of their race had led them at the beginning of the Christian era was fitly expressed by those Athenians, who erected near the Areopagus the "altar on which was written, 'To the Unknown God.'"[86] The opinion (for in most cases it did not amount to a conviction) that there was an Unknown (or even, as many thought, an Unknowable) Divinity of some sort, which might account for the phenomena of the world, and which might be the truth behind the vagaries of the anthropomorphic polytheism, was as far as Greek thought had led men at the period with which we have to do. Their θεός was really nothing more than Mr. Herbert Spencer's "Unknowable,"--a mysterious "force," to which everything was referred which could not be accounted for on the basis of scientific principles.