5. The conditions of a phenomenon often do resemble the phenomenon itself, e.g. in cases of Motion, Contagion, Feelings; but it is a Fallacy to suppose that _they must or probably will_. By this fancied law men guided their conjectures. Thus, the _Doctrine of Signatures_ was, that substances showed their uses as medicines by external resemblance, either to their supposed effect, or to the disease. So, the Cartesians, and even Leibnitz, argued, that nothing physical but previous motion could account for motion, explaining the human body's voluntary motions by Nervous Vibrations or by Animal Spirits. Hence, too, the inference that there is a correspondence between the physical qualities of the cause, and like or like-named ones, either of the phenomenon (e.g. between sharp particles and a sharp taste), or of its effects (e.g. between the redness of Mars, and fire and slaughter as results of that planet's influence). In metaphysics, the Epicureans' doctrine of _species sensibiles_, and the moderns' of _perception through ideas_, arose from this fallacy (combined with another, viz. that a thing cannot act where it is not). Again, the conditions of a thing are sometimes spoken of even as though they were the thing itself. Thus, in the Novum Organon, heat (i.e. really the conditions of the feeling of it) is called a kind of motion; and Darwin, in his Zoonomia, after describing _idea_ as a kind of _notion of external things_, defines it as _a motion of the fibres_. Cousin says: 'Tout ce qui est vrai de l'effet est vrai de la cause,' though, the reverse _might_ be true; and Coleridge affirms, as _an evident truth_, that mind and matter, as having no common property, cannot act on each other. The same fallacy led Leibnitz to his _pre-established harmony_, and Malebranche to his _occasional causes_. So, Cicero argues that mental pleasures, if arising from the bodily, could not, as they do, exceed their cause; and Descartes, that the Efficient Cause must have all the perfections of the effect. Conversely Descartes, too, and persons who assail, e.g. the Principle of Population by reference to Divine benevolence (thus implying that, because God is perfect, therefore what _they_ think perfection must obtain in nature), assume that effects must resemble their causes. CHAPTER IV. FALLACIES OF OBSERVATION. A fallacy of Observation (the first of the three fallacies of Proof) may be either negative or positive.