"The supreme and all-pervading law of nature, the true and only cosmological law, is, in my opinion, _the law of substance_; its discovery and establishment is the greatest intellectual triumph of the nineteenth century, in the sense that all other known laws of nature are subordinate to it. Under the name of 'law of substance' we embrace two supreme laws of different origin and age—the older is the chemical law of the 'conservation of matter,' and the younger is the physical law of the 'conservation of energy.' It will be self-evident to many readers, and it is acknowledged by most of the scientific men of the day, that these two great laws are essentially inseparable" (p. 75). "The conviction that these two great cosmic theorems, the chemical law of the persistence of matter and the physical law of the persistence of force, are fundamentally one, is of the utmost importance in our monistic system. The two theories are just as intimately united as their objects—matter and force or energy. Indeed, this fundamental unity of the two laws is self-evident to many monistic scientists and philosophers, since they merely relate to two different aspects of one and the same object, the _cosmos_" (p. 76). "I proposed some time ago to call it the 'law of substance,' or the 'fundamental cosmic law'; it might also be called the 'universal law,' or the 'law of constancy,' or the 'axiom of the constancy of the universe.' In the ultimate analysis it is found to be a necessary consequence of the principle of causality" (p. 76). I criticise these utterances below, and I also quote extracts bearing on the subject from Professor Huxley in Chapter IV.; but meanwhile Professor Haeckel is as positive as any Positivist, and runs no risk of being accused of Solipsism:— "Our only real and valuable knowledge is a knowledge of nature itself, and consists of presentations which correspond to external things."... "These presentations we call _true_, and we are convinced that their content corresponds to the knowable aspect of things. We _know_ that these facts are not imaginary, but real" (p. 104). He also tends to become sentimental about the ultimate reality as he perceives it, and tries to construct from it a kind of religion:— "The astonishment with which we gaze upon the starry heavens and the microscopic life in a drop of water, the awe with which we trace the marvellous working of energy in the motion of matter, the reverence with which we grasp the universal dominance of the law of substance throughout the universe—all these are part of our emotional life, falling under the heading of 'natural religion'" (p. 122).