For my part, I am convinced of the profound importance of the unifying "law of substance," as an expression of the inseparable connection in reality of two laws which are only separated in conception. That they were not originally taken together and their unity recognized from the beginning is merely an accident of the date of their respective discoveries. The earlier and more accessible chemical law of the persistence of matter was detected by Lavoisier in 1789, and, after a general application of the balance, became the basis of exact chemistry. On the other hand, the more recondite law of the persistence of force was only discovered by Mayer in 1842, and only laid down as the basis of exact physics by Helmholtz. The unity of the two laws--still much disputed--is expressed by many scientists who are convinced of it in the formula: "Law of the persistence of matter and force." In order to have a briefer and more convenient expression for this fundamental thought, 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.[24] The first thinker to introduce the purely monistic conception of substance into science and appreciate its profound importance was the great philosopher Baruch Spinoza; his chief work appeared shortly after his premature death in 1677, just one hundred years before Lavoisier gave empirical proof of the constancy of matter by means of the chemist's principal instrument, the balance. In his stately pantheistic system the notion of the _world_ (the universe, or the cosmos) is identical with the all-pervading notion of God; it is at one and the same time the purest and most rational _monism_ and the clearest and most abstract _monotheism_. This universal substance, this "divine nature of the world," shows us two different aspects of its being, or two fundamental attributes--matter (infinitely _extended_ substance) and spirit (the all-embracing energy of _thought_). All the changes which have since come over the idea of substance are reduced, on a logical analysis, to this supreme thought of Spinoza's; with Goethe I take it to be the loftiest, profoundest, and truest thought of all ages. Every single object in the world which comes within the sphere of our cognizance, all individual forms of existence, are but special transitory forms--_accidents_ or _modes_--of substance. These modes are material things when we regard them under the attribute of _extension_ (or "occupation of space"), but forces or ideas when we consider them under the attribute of _thought_ (or "energy"). To this profound thought of Spinoza our purified monism returns after a lapse of two hundred years; for us, too, matter (space-filling substance) and energy