Ought we to designate these constant alterations in the inner vital conditions as “stimuli”? Usage in this connection has already answered in the negative, by applying to them the word “_development_.” And this use is in a certain sense justified. Let us imagine an organism or any other object for the purpose of investigation as isolated from its surroundings. This conception, which we have already stated, proves untenable on closer analysis, but it, however, is based on the nature of the methods of human observation and is indispensable for practical use within certain limits. Then the inner vital conditions belong to the organism, the external to the medium. They differ in so far that the external vital conditions can exist permanently without alteration, that is, independently of the development of living systems, whilst the inner vital conditions of every living organism continuously and progressively undergo alteration. In this sense, but only in this, there is evidently a difference between the inner and outer vital conditions, which permits a separation of the two groups. But we should always bear in mind that this separation cannot be sharply defined. On the same basis we assume that the organism for purposes of study is separated from its surroundings as an independent system, which leads us in consequence to contrast the alterations in the internal with those in the external vital conditions, in which we designate the first as processes of _development_, the latter as stimuli. This distinction, as all differentiations and separations in nature, gives us only a practical working basis. In this way we confine the conception of the stimulus to all alterations in the external vital conditions of a living system, considered as isolated. This view does not exclude the fact that stimuli can also occur and act within an organism. If a nervous impulse is conducted from the cerebral cortex through the pyramidal tract to a skeletal muscle, this impulse acts upon the muscle cells as a stimulus. Although the explosion of the impulse is an alteration within the body, nevertheless, as far as the muscle is concerned, it may be looked upon as an external vital condition, therefore as a stimulus. As the conception of stimulus involves the relation to a given state, it likewise involves at the same time the relation to a given living system, upon which it acts from the exterior. What is the value then of all this theoretical discussion?