All processes--_i.e._ the entire play of world-events--fall into two great classes: those _that are maintained_, dead processes, and those _that maintain themselves_, living processes; the latter presenting themselves, on the one hand, as processes of combustion, as flame, and on the other as processes of alimentation, as living beings. All dead processes can be interpreted or read as falls. Their type is the falling stone. A stone does not fall because of an indwelling force that causes its falling; it only falls because it has previously been raised, because between it and the surface of the earth there exists a difference of tension. Its fall thus signifies that force must have been present, in the sense that it must _previously_ have been active; for otherwise the difference in position of stone and surface of the earth could never have come about. When physics interprets the fall of the stone in differing fashion--namely, by having it caused by the attractive force of the earth’s surface in action during the fall--this is purely a working hypothesis, advanced solely in the interest of a uniform physical world-theory. To much the same effect as the falling stone, every physical happening without exception is to be interpreted or read, whether it concern mechanical, chemical, thermal, electrical, magnetic, or any other such-like phenomena. All alike are to be taken as falls from places of higher to places of lower tension. The import of each and all is only that forces, actuating impulsions, _must once have been present_. In each case we really have to do not with actions but with reactions. The proof that no actual forces are here at work is to be found in the fact that the process ceases so soon as the differences of tension are adjusted. This world of reactions is the given province of all science. Science, because bent upon furnishing demonstration, has a title to existence only where there is nothing that is not perceptible to sense. Where there are actual living processes, there _actual forces must be present_. A force, however, can never be perceptible to sense; for everything perceptible to sense necessitates the question as to its adequate cause--that is, as to the force in virtue of which it exists. Where there are dead, re-actual processes, there forces are not in action themselves, and hence force is not a real but only a conceptual necessity, a mere logical presumption. Hence also in the interpretation of this re-actual world, it is always possible to slur over, to eliminate the question as to actual forces, and to replace these latter by the various differences of tension, of potentiality, and thus remain wholly within the domain of the sensible.