There are always numerous factors which bring about the state or process. Would it be possible to distinguish which of these particular conditions is of the greatest importance? First of all, it must here be taken into consideration that the importance of a condition is not one which is capable of increase or decrease, for the simple reason that necessity, which forms an essential component of the conception of cause cannot be varied. A factor cannot be _more_ than necessary for the existence of a state or the taking place of a process. If, however, it is less than necessary, then it is not necessary at all, and the state or process exists also without it, that is to say, the factor is not a condition. In other words: _all conditions for a state or process are of equal value for its existence, as they are all necessary_. If one attempts to prove by means of concrete examples this statement obtained by purely logical deduction--a control which, considering the experimental nature of modern thought, never should be neglected even in the simplest of reasoning--it might appear that an objection could still be made against its general validity. From various instances it might be concluded that there are conditions, which as such are not absolutely necessary for a state or process, but can be replaced by other factors. An example may serve to make this clear. I pour diluted hydrochloric acid on powdered carbonate of sodium, and carbon dioxide is set free. The addition of hydrochloric acid is here a condition for the liberation of the carbon dioxide. Without the presence of the hydrochloric acid the process does not occur. Nevertheless I can substitute diluted sulphuric acid for the hydrochloric acid. Here it would appear that one condition can be replaced by another. But one must not be deceived. A closer observation soon shows that the process has not been sufficiently analyzed if we look upon the addition of hydrochloric acid as a condition for the liberation of carbon dioxide. It is not the presence of hydrochloric acid or sulphuric acid, as such, which is a condition for the process, but rather the separation of the sodium atoms from their combinations with the oxygen in the molecule of the carbonate. This reaction can occur as a partial component in very different complexes of processes. Or to quote another example, taken from the subject with which we are especially here concerned. I allow an induction shock to act on the nerve of a nerve muscle preparation of the frog. The muscle contracts. The electric stimulus is the condition for the muscle contraction. But I can substitute for the induction shock a mechanical stimulus by sudden pressure of the nerve. The muscle again contracts. The analysis again shows that the induction shock as such was not the condition for the muscle contraction, but the excitation of the nerve which it produced and which is conducted as a specific impulse to the muscle. This excitation of the nerve can,