There is especially one circumstance pertaining to all these transformations that merits a closer attention. If we remember that all the material used in a product of art is taken from nature, and besides that, all the processes in making and shaping the raw material are carried out through the employment of natural laws, we might still ask the question, why physical forces should not enter spontaneously into the necessary artificial combinations for producing this result. Until we have pointed out the quality in matter which prevents this, we have not completely demonstrated the inability of natural forces to build an engine spontaneously. This quality has been named _vis inertiae_, the inertia of matter, one of the most important natural laws that exist. What does this law teach us? It says that matter cannot itself change its condition. If a body is in motion it can never come to rest unless another force at least equal to the primary opposes the motion. If it be at rest, it cannot impart motion unto itself; energy, applied from without, is necessary. Inertia keeps the earth moving around the sun; a stone thrown into the air would proceed everlastingly with its initial velocity if the attraction of the earth did not interfere. Because of this quality, then, matter remains in its natural equilibrium. An engine would never be built because the ore would stay in the mountains and the metals forever remain in their compounds. Every product of art requires a foreign interference in the material world; matter, in consequence of its inertia, presents a determined and often very energetic resistance to such an intervention. Exactly the same reasons that prevent natural forces from building a steam engine, cause also their inability to produce an organism, and this in a much higher degree because the organism is in a still fuller sense a product of art. The organic building material, instead of being plates and ingots of iron, copper, lead, etc., consists of carbon, hydrogen, sulphur, phosphorus, chlorin, potassium, sodium, magnesia, etc., or both metals and metalloids of which the former, on account of their negative, and the latter because of their positive qualities cannot exist in a free state. From the minerals found in nature these substances must be extracted for organic purposes. The elements are different, but otherwise we may verbally repeat in regard to organic substance what has been previously said about the steam engine.