On the other hand, the conception of a personal immortality cannot be maintained. If this idea is still widely held, the fact is to be explained by the physical law of inertia; for the property of persistence in a state of rest exercises its influence in the region of the ganglion-cells of the brain, as well as in all other natural bodies. Traditional ideas handed down through many generations are maintained with the greatest tenacity by the human brain, especially if, in early youth, they have been instilled into the childish understanding as indisputable dogmas. Such hereditary articles of faith take root all the more firmly, the further they are removed from a rational knowledge of nature, and enveloped in the mysterious mantle of mythological poesy. In the case of the dogma of personal immortality, there comes into play also the interest which man fancies himself to have in his individual future existence after death, and the vain hope that in a blessed world to come there is treasured up for him a compensation for the disappointed hopes and the many sorrows of his earthly life. It is often asserted by the numerous advocates of personal immortality that this dogma is an innate one, common to all rational men, and that it is taught in all the more perfect forms of religion. But this is not correct. Neither Buddhism nor the religion of Moses originally contained the dogma of personal immortality, and just as little did the majority of educated people of classical antiquity believe it, at any rate during the highest period of Greek culture. The monistic philosophy of that time, which, five hundred years before our era, had reached speculative heights so remarkable, knew nothing of any such dogma. It was through Plato and Christ that it received its further elaboration, until, in the Middle Ages, it was so universally accepted, that only now and then did some bold thinker dare openly to gainsay it. The idea that a conviction of personal immortality has a specially ennobling influence on the moral nature of man, is not confirmed by the gruesome history of mediaeval morals, and as little by the psychology of primitive peoples.[17] If any antiquated school of purely speculative psychology still continues to uphold this irrational dogma, the fact can only be regarded as a deplorable anachronism. Sixty years ago such a doctrine was excusable, for then nothing was accurately known either of the finer structure of the brain, or of the physiological functions of its separate parts; its elementary organs, the microscopic ganglion-cells, were almost unknown, as was also the cell-soul of the Protista; very imperfect ideas were held as to ontogenetic development, and as to phylogenetic there were none at all.