egregious flaw in the reasoning, namely, that of _cum hoc, ergo propter hoc_ (or the assumption of causation from mere coexistence); and this, too, in its very worst form. For it is not _cum hoc solo, ergo propter hoc_, which would in many cases supply a presumptive proof by induction, but _cum hoc, et plurimis aliis, ergo propter hoc_! Shell, of some kind or other, is common to the whole order of testacea, but it would be absurd to define the _vis vitæ_ of testaceous animals as existing in the shell, though we know it to be the constant accompaniment, and have every reason to believe the constant effect, of the specific life that acts in those animals. Were we (_argumenti __ causá_) to imagine shell coextensive with the organized creation, this would produce no abatement in the falsity of the reasoning. Nor does the flaw stop here; for a physiological, that is a real, definition, as distinguished from the verbal definitions of lexicography, must consist neither in any single property or function of the thing to be defined, nor yet in all collectively, which latter, indeed, would be a history, not a definition. It must consist, therefore, in the _law_ of the thing, or in such an _idea_ of it, as, being admitted, all the properties and functions are admitted by implication. It must likewise be so far _causal_, that a full insight having been obtained of the law, we derive from it a progressive insight into the necessity and _generation_ of the phenomena of which it is the law. Suppose a disease in question, which appeared always accompanied with certain symptoms in certain stages, and with some one or more symptoms in all stages—say deranged digestion, capricious alternation of vivacity and languor, headache, dilated pupil, diminished sensibility to light, &c.—Neither the man who selected the one constant symptom, nor he who enumerated all the symptoms, would give the scientific definition _talem scilicet, quali scientia fit vel datur_, but the man who at once named and defined the disease hydrocephalus, producing pressure on the brain. For it is the essence of a scientific definition to be causative, not by introduction of imaginary somewhats, natural or supernatural under the name of causes, but by announcing the law of action in the particular case, in subordination to the common law of which all the phenomena are modifications or results.