Like all science, biology is _realistic_--that is to say, it regards its object, the organisms, as really existing things, the features of which are to an extent knowable through our senses (_sensorium_) and organ of thought (_phronema_). At the same time, we know that these cognitive organs, and the knowledge they bring us, are imperfect, and that there may be other features of organisms that lie beyond our means of perception altogether. But it by no means follows from this that, as our idealist opponents say, the organisms (and all other things) exist only in our mind (in the images in our cortex). Our pure monism (or hylozoism) agrees with realism in recognizing the unity of being of each organism, and denying that there is any essential distinction between its knowable phenomenon and its internal hidden essence (or noumenon), whether the latter be called, with Plato, the eternal "idea," or, with Kant, the "thing in itself." Realism is not identical with materialism, and may even be definitely connected with the very opposite, dynamism or energism. As realism generally coincides with monism, so idealism is usually identical with dualism. The two most influential representatives of dualism, Plato and Kant, said that there were two totally distinct worlds. Nature, or the empirical world, is alone accessible to our experience, while the spiritual or transcendental world is not. The existence of the latter is known to us only by the emotions or by practical reason; but we can have no idea of its nature. The chief error of this theoretical idealism is the assumption that the soul is a peculiar, immaterial being, immortal and endowed with _a priori_ knowledge. The physiology and ontogeny of the brain (together with the comparative anatomy and histology of the phronema) prove that the soul of man is, like that of all other vertebrates, a function of the brain, and inseparably bound up with this organ. Hence this idealist theory of knowledge is just as inconsistent with realistic biology as is the psycho-physical parallelism of Wundt or the psychomonism of more recent physiologists, which in the end issues in a complete dualism of body and mind. It is otherwise with _practical_ idealism. When this presents the symbols or ideals of a personal God, an immortal soul, and the free-will as ethical stimuli, and uses them for their pedagogical worth in the education of the young, it may have a good influence for a time, which is independent of their theoretical untenability.