The Comparative Method

The Encyclopedia, or a Systematic Dictionary of the Sciences, Arts, and Crafts was published in France from 1751-1772, co-edited by Denis Diderot and Jean D'Alembert. Their intent was to provide knowledge of the new sciences and the associated mechanical arts to the public, in line with Enlightenment ideals on rationalism and secular knowledge. In this extract from the article on "Histoire Naturelle" [Natural History] in 1765, the unknown author describes the value of broader comparison, beyond simple description and cataloging, for discovery of the general laws of nature:

"The description of nature’s works forms the foundation of its history; this is the only way to distinguish each one in particular and to give a correct idea of their formation. There are two types of descriptions: incomplete and complete. In the first type, one’s only goal is to specify the characteristics of each thing until it is distinguished from others; this description is nothing more than a designation, most often quite dubious, no matter the method used to express the distinctive characteristics of each object. The works of nature are too numerous and too varied—most are only differentiated by such imperceptible nuances that one cannot hope to represent them with words, and such a portrait is most often inaccurate. As proof of this fact, one need only glance at the systems of nomenclature that have been constructed in Natural history —they are all flawed. Nevertheless, if we were to skim through the list of the authors of those systems, we cannot doubt that they would have produced accurate ones if it were possible to attain this degree of perfection in descriptions whose only object is nomenclature and which are concerned only with parts of each object. Complete descriptions reveal all objects in their entirety and not only make them unequivocally recognizable but indicate the relation between their constituent parts.
In this perspective, descriptions include the interior parts of each object as well as the exterior; they express, as much as possible, the proportions of shape and weight, the dimensions of area, and all qualities that can provide a correct idea of the formation of the principal parts of each thing. By such descriptions, one can compare one object to another and evaluate the resemblance and dissimilarity found in their conformation. One can recognize the different means that nature employs to produce the same effect, and one arrives at general conclusions, which are the most precious facts for Natural history.
The naturalist considers something only to compare it to other things; he prefers to observe the characteristics of each thing that distinguishes it from others, and he does his best to see the progression of nature in its works. The anatomist, on the contrary, contemplates each thing in and of itself. He closely observes each one of its parts in order to discover those that are least apparent, and he uses all of his abilities to identify the first material agents and all of the mechanisms that nature uses to move animate bodies.
Thus far, Anatomy has had nearly no focus apart from man, who is clearly its principal consideration, but the human body does not contain all the types of mechanisms present in the animal kingdom. In animals, there are different configurations than in man, and they have more developed parts. By comparing them to each other and to man, we will better understand man in particular and the mechanics of nature in general. This is the grand object of Comparative anatomy, which has a more immediate relation to Natural history than simple Anatomy. From the latter, one can only make observations relating to detail, whereas the former yields general results and facts that constitute the corpus of the Natural history of animals. "

With thanks to The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. Translated by Marc Olivier and Valerie Mariana. Ann Arbor: Michigan Publishing, University of Michigan Library, 2015. <http://hdl.handle.net/2027/spo.did2222.0000.189>.