But the confusion between forces of Nature and causes is as frequent as it is detrimental to clearness of thought. It seems indeed as though no one had accurately defined the difference between these conceptions before me, however great may have been the urgency for such a distinction. Not only are forces of Nature turned into causes by such expressions as, 'Electricity, Gravity, &c., are the _cause_ of so-and-so,' but they are even often turned into effects by those who search for a cause for Electricity, Gravity, &c. &c., which is absurd. Diminishing the number of the forces of Nature, however, by reducing one to another, as for instance Magnetism is in our days reduced to Electricity, is a totally different thing. Every _true_, consequently really primary force of Nature--and every fundamental chemical property belongs to these forces--is essentially a _qualitas occulta_, _i.e._ it does not admit of physical, but only of metaphysical explanation: in other words, of an explanation which transcends the world of phenomena. No one has carried this confusion, or rather identification, of causes with forces of Nature further than Maine de Biran in his "Nouvelles considérations des rapports du physique au moral," for it is essential to his philosophy. It is besides remarkable, that when he speaks of causes, he rarely uses the word _cause_ alone, but almost always speaks of _cause ou force_, just as we have seen Spinoza above (§ 8) write _ratio sive causa_ no less than eight times in the same page. Both writers are evidently conscious that they are identifying two disparates, in order to be able to make use of the one or the other, according to circumstances; for this end they are obliged to keep the identification constantly before their readers' mind.-- [66] See "Die Welt a. W. u. V." vol. i. § 26, p. 153 of the 2nd, and p. 160 of the 3rd edition. Now Causality, as the director of each and every change, presents itself in Nature under _three_ distinct forms: as _causes_ in the strictest acceptation of the word, as _stimuli_, and as _motives_. It is just upon this difference that the real, essential distinction between inorganic bodies, plants, and animals is based, and not upon external, anatomical, let alone chemical, distinctions. A _cause_, in its narrowest sense, is that upon which changes in the _inorganic_ kingdom alone ensue: those changes, that is to say, which form the theme of Mechanics, Physics, and Chemistry. Newton's third fundamental law, "Action and reaction are equal to one another," applies exclusively to this cause, and enunciates, that the state which precedes (the cause) undergoes a change equivalent to that produced by it (the effect). In this form of causality alone, moreover, does the degree of the effect always exactly correspond to the degree of the cause, so as to enable us accurately to calculate the one by means of the other.