To the connexion here asserted, of theory and nomenclature, we have the testimony of the author of the oxygen theory. In the Preface to his _Chemistry_, Lavoisier says:--'Thus while I thought myself employed only in forming a Nomenclature, and while I proposed to myself nothing more than to improve the chemical language, my work transformed itself by degrees, without my being able to prevent it, into a Treatise on the Elements of Chemistry.' And he then proceeds to show how this happened. It is, however, mainly through the progress of Natural History in modern times, that philosophers have been led to see the importance and necessity of new terms in expressing new truths. Thus Harvey, in the Preface to his work on Generation, says:--'Be not offended if in setting out the History of the Egg I make use of a new method, and sometimes of unusual terms. For as they which find out a new plantation and new shores call them by names of their own coining, which posterity afterwards accepts and receives, so those that find out new secrets have good title to their compellation. And here, methinks, I hear Galen advising: If we consent in the things, contend not about the words.' The Nomenclature which answers the purposes of Natural History is a Systematic Nomenclature, and will be further considered under the next Aphorism. But we may remark, that the Aphorism now before us governs the use of words, not in science only, but in common language also. Are we to apply the name _fish_ to animals of the whale kind? The answer is determined by our present rule: we are to do so, or not, accordingly as we can best express true propositions. If we are speaking of the internal structure and physiology of the animal, we must not call them _fish_; for in these respects they deviate widely from fishes: they have warm blood, and produce and suckle their young as land quadrupeds do. But this would not prevent our speaking of the _whale-fishery_, and calling such animals _fish_ on all occasions connected with this employment; for the relations thus arising depend upon the animal's living in the water, and being caught in a {287} manner similar to other fishes. A plea that human laws which mention fish do not apply to whales, would be rejected at once by an intelligent judge. [A bituminiferous deposit which occurs amongst the coal measures in the neighbourhood of Edinburgh was used as coal, and called 'Boghead Cannel Coal.' But a lawsuit arose upon the question whether this, which geologically was not _the coal_, should be regarded in law as _coal_. The opinions of chemists and geologists, as well as of lawyers, were discrepant, and a direct decision of the case was evaded.[25\4]] [Note 25\4: Miller's _Chemistry_, iii. 98.] APHORISM IX. _In the Classificatory Sciences, a Systematic Nomenclature is necessary; and the System and the Nomenclature are each essential to the utility of the other._