Chapter III, 2

Let us now bring our general rule—which is certain prior to experience and therefore valid without exception for every possible experience—closer to experience itself, and consider the real objects given in it, to whose possible changes our rule refers. We soon notice certain profound, fundamental differences among these objects, according to which they have long been classified: some are inorganic, that is, lifeless; others organic, that is, living; and these in turn are either plants or animals. Although animals are essentially alike and conform to their concept, we find them arranged in an extraordinarily varied and finely graduated scale of perfection, from those still closely related to plants and difficult to distinguish from them, up to the most fully developed, which correspond most perfectly to the concept of an animal. At the summit of this scale we see the human being—ourselves.

If, without allowing this diversity to mislead us, we consider all these beings solely as objective, real objects of experience, and accordingly proceed to apply our law of causality, established prior to experience as a condition of all possible experience, to whatever changes may occur in such beings, we will find that experience everywhere conforms to the law whose certainty precedes it. Yet the great diversity in the nature of all these objects of experience, recalled above, is matched by a corresponding modification in the way causality asserts its authority over them. More precisely: corresponding to the threefold distinction between inorganic bodies, plants, and animals, the causality governing all their changes also appears in three forms: as cause in the narrowest sense of the word, as stimulus, or as motivation—without this modification impairing in the slightest either its validity prior to experience or, consequently, the necessity of the result that it establishes.

Cause in the narrowest sense of the word is that through which all mechanical, physical, and chemical changes in objects of experience occur. It is everywhere characterized by two features. First, Newton’s third fundamental law applies to it: “Action and reaction are equal.” That is, the preceding state, called the cause, undergoes a change equal to that of the succeeding state, called the effect.—Second, in accordance with Newton’s second law, the degree of the effect always corresponds exactly to the degree of the cause, so that an increase in the latter produces an equal increase in the former. Thus, once the mode of action is known, the degree of the effect can immediately be known, measured, and calculated from the degree of intensity of the cause, and conversely. In applying this second feature empirically, however, we must not confuse the actual effect with its visible manifestation. For example, in compressing a body, we must not expect its volume to go on diminishing in proportion to the increase in the compressing force. For the space into which the body is forced continually diminishes, and resistance consequently increases. Although here too the actual effect, which is the increase in density, really does grow in proportion to the cause, as Mariotte’s law states, this does not apply to its visible manifestation. Again, heat supplied to water will warm it up to a certain point, but beyond that point will only make it evaporate rapidly. Here, however, the same proportion once again holds between the degree of the cause and that of the effect; and so it is in many cases. Causes in this narrowest sense bring about the changes in all lifeless, that is, inorganic bodies. Knowledge and the presupposition of causes of this kind guide the investigation of all the changes studied by mechanics, hydrodynamics, physics, and chemistry. To be determined exclusively by causes of this kind alone is therefore the distinctive and essential characteristic of an inorganic or lifeless body.

The second kind of cause is the stimulus: a cause that, first, does not itself undergo a reaction proportionate to its action, and, second, has no uniform relation whatever between its intensity and the intensity of its effect. Here, therefore, the degree of the effect cannot be measured and determined in advance from the degree of the cause. Rather, a small increase in the stimulus may cause a very great increase in the effect, or, conversely, may entirely cancel the previous effect and even produce its opposite. For example, plants can, as we know, be driven to extraordinarily rapid growth by warmth or by lime mixed into the soil, since these causes act as stimuli to their vital force. But if the proper degree of stimulus is exceeded even slightly, the result, instead of intensified and accelerated life, will be the plant’s death. Likewise, wine or opium can stimulate and considerably heighten our mental powers; but if the proper measure of stimulus is exceeded, the result will be precisely the opposite.—Causes of this kind, then, namely stimuli, determine all changes in organisms as such. All changes and developments in plants, and all purely organic and vegetative changes or functions in animal bodies, occur in response to stimuli. In this way light, warmth, air, nourishment, every drug, every touch, fertilization, and so on act upon them.—While the life of animals also has an entirely different sphere, of which I will speak shortly, the whole life of plants, by contrast, proceeds exclusively in response to stimuli. All their assimilation, growth, striving with their crowns toward the light and with their roots toward better soil, their fertilization, germination, and so on are changes in response to stimuli. In a few genera, these are joined by a distinctive rapid movement that likewise occurs only in response to stimuli, but on account of which they are called sensitive plants. As is well known, these are chiefly Mimosa pudica, Hedysarum gyrans, and Dionaea muscipula. To be determined exclusively and without exception by stimuli is the defining character of the plant. A plant, therefore, is any body whose distinctive movements and changes, in keeping with its nature, always and exclusively occur in response to stimuli.

Let us now bring our general rule closer to experience itself. The rule is certain a priori and therefore holds without exception for all possible experience. We will examine the real objects found in experience, whose possible changes our rule concerns. We soon notice some deep, basic differences among them. These have long been used to classify them. Some are inorganic, that is, lifeless. Others are organic, that is, living. The latter are either plants or animals. Animals are essentially alike and fit their shared concept. Yet we find them arranged in a very varied and finely graded scale of perfection. It starts with animals still closely related to plants and hard to distinguish from them. It rises to the most fully developed animals, which most perfectly fit the concept of an animal. At the top of this scale we see human beings—ourselves.

Let us not be distracted by this variety. Instead, let us consider all these beings simply as objective, real objects of experience. We can then apply the law of causality to any changes they may undergo. This law is established a priori as a condition of any experience being possible. We will find that experience everywhere agrees with the law that is certain a priori. But the great differences in the nature of these objects of experience, just recalled, are matched by a corresponding change in how causality governs them. More precisely, there are three forms of the causality that governs all their changes. These correspond to the threefold division into inorganic bodies, plants, and animals. They are cause in the narrowest sense of the word, stimulus, and motivation. These different forms do not in the least weaken causality's a priori validity, or the necessity of the result that it imposes.

A cause in the narrowest sense of the word brings about all mechanical, physical, and chemical changes in objects of experience. It always has two distinguishing features. First, Newton's third fundamental law applies to it: "Action and reaction are equal." That is, the earlier state, called the cause, undergoes a change equal to that in the later state, called the effect.—Second, in keeping with Newton's second law, the degree of the effect always exactly matches the degree of the cause. So strengthening the cause brings an equal strengthening of the effect. Once we know how the effect works, we can know, measure, and calculate its degree from the intensity of the cause, and the other way around. But when applying this second feature to experience, we must not confuse the actual effect with its visible appearance. For example, when compressing a body, we must not expect its size to keep shrinking in proportion to the increase in the compressing force. The space into which we force the body keeps getting smaller, so resistance increases. Here too the actual effect, which is an increase in density, does grow in proportion to the cause, as Mariotte's law states. But this does not apply to the effect's visible appearance. Again, heat supplied to water will raise its temperature up to a certain point. Beyond that point it will only cause rapid evaporation. In evaporation, however, the same proportion between the degree of the cause and that of the effect applies again. The same holds in many cases. Causes in this narrowest sense bring about the changes in all lifeless, that is, inorganic bodies. Knowing and assuming causes of this kind guides our study of all the changes that are the subject of mechanics, hydrodynamics, physics, and chemistry. Being determined exclusively by causes of this kind is therefore the true and essential mark of an inorganic or lifeless body.

The second kind of cause is the stimulus. First, this is a cause that does not itself undergo a reaction proportional to its action. Second, there is no proportional relation at all between its intensity and the intensity of its effect. We therefore cannot measure or predict the degree of the effect from the degree of the cause. Instead, a small increase in the stimulus can cause a very large increase in the effect. Or, the other way around, it can completely cancel the earlier effect and even produce the opposite one. For example, as everyone knows, plants can be made to grow extraordinarily fast by warmth or by lime mixed into the soil. These causes act as stimuli to their vital force. But if the proper degree of stimulus is exceeded even a little, the result will not be stronger, faster life. It will be the plant's death. Likewise, we can use wine or opium to heighten our mental powers and increase them considerably. But if the proper amount of stimulus is exceeded, the result will be exactly the opposite.—This kind of cause, then, namely stimuli, determines all the changes in organisms as such. All the changes and developments of plants occur in response to stimuli. So do all the purely organic and vegetative changes, or functions, of animal bodies. Light, heat, air, food, every drug, every touch, fertilization, and so on affect them in this way.—Animal life also has an entirely different sphere, which I will discuss in a moment. But the whole life of plants proceeds exclusively through stimuli. All their assimilation, growth, reaching toward the light with their crowns and toward better soil with their roots, their fertilization, germination, and so on are changes in response to stimuli. A few particular genera also have a distinctive rapid movement. This too occurs only in response to stimuli, though it has earned them the name sensitive plants. As is well known, the main examples are Mimosa pudica, Hedysarum gyrans, and Dionaea muscipula. Being determined exclusively and without exception by stimuli is the defining feature of a plant. Thus a plant is any body whose characteristic movements and changes, suited to its nature, always occur exclusively in response to stimuli.