The 17th century ushered in some new ways of thinking about knowledge. One of the 17th-century philosophers who studied the nature of knowledge (among many other things) was René Descartes (1596-1650), who concerned himself with the nature of knowledge (among many other things).








Descartes is perhaps best known to us as the writer of “I think, therefore I am.” As a mathematician, he also established some of the principles of modern algebra and symbolic logic.

Descartes’ famous Wax Argument is of particular interest to us as taxonomists. In that writing, he argued that to classify something just according to its characteristics is a questionable approach. He considered a piece of wax, whose characteristics change as it becomes warm. Think about it.





Descartes used the Wax Argument to support his position that perception is unreliable, and that we acquire knowledge through deduction.

Next is John Locke (1632-1704), an English physician and philosopher.








In An Essay Concerning Human Understanding, he maintained that at birth, the human mind is a blank slate, and that all knowledge is acquired through perception and experience.






Locke identified three kinds of knowledge, differing both in the way the knowledge is acquired and in the degree of certainty they provide:

  • Intuitive knowledge provides the highest degree of certainty.
  • Demonstrative knowledge provides less certainty.
  • Sensitive knowledge provides the lowest degree of certainty, which can be thought of as probability.

Book III of the Essay is devoted to an exploration of the relationships between words and concepts, an area of particular interest to us in connection with controlled vocabularies. As for classification, and biological taxonomy in particular, Locke starts from the old idea of “essences”. Watch for his comments relating classification to characteristics, and on the arbitrariness that is sometimes unavoidable in classification:

“Academic wrangling about genus and species has had the effect of almost entirely suppressing that original meaning of ‘essence’. Instead of referring to the real constitutions of things, essences these days are usually thought of in a second way, in which they are connected with the artificial constitution of genus and species. Real constitutions are ones that are laid down in the things themselves; artificial ones are products of human artifice, that is, of human classificatory procedures·. When people talk in this way, they assume that each sort of things has a real constitution; and it is unquestionably true that any collection of simple ideas [‘qualities’] that regularly go together must be based on some real constitution. But the fact remains that when things are classified into sorts or species, and named accordingly, what we go by are the abstract ideas with which we have associated those names. The essence of each genus or sort—that is, what fixes the sort, what determines membership in it—is just the abstract idea that the general name stands for. This, we shall find, is how ‘essence’ is mostly used. These two sorts of essences could reasonably be called the real and the nominal essence respectively.”

“Nominal essences are tied to names. Whether a given thing x is to be described by a given general name depends purely on whether x has the essence that makes it conform to the abstract idea that the name is associated with.”

“There are two opinions about the real essences of bodies. Some people think there is a certain limited number of real essences according to which all natural things are made. Each particular thing, they believe, exactly fits one of these essences, and thus belongs to one species. These folk use the word ‘essence’ without knowing what essences are. Others have a more reasonable view: according to them, the essence of a natural thing is the real but unknown constitution of its imperceptible parts, from which flow the perceptible qualities on the basis of which we classify things into sorts under common names. The former of these opinions, which takes essences to be a certain number of forms or moulds into which all natural things are poured (so to speak) has created great confusion in the knowledge of natural things. In every animal species, births frequently occur, and … sometimes produce … strange products …; and all this poses problems for this hypothesis about real essences …. Even apart from those difficulties, the mere fact that these ‘first-opinion’ real essences can’t be known means that they are useless to us in classifying things, although they are supposed to mark off the real boundaries of the species! In our thoughts about classification, then, we ought to set these supposed real essences aside—and, for the same reason, set aside ‘second-opinion’ real essences as well—and content ourselves with knowable essences of sorts or species. When we think the matter through, we shall see that these are, as I have said, nothing but the abstract complex ideas with which we have associated separate general names.”

I think that’s enough to chew on as far as the 17th century is concerned. In the next installment, we’ll look at biological classification and other developments in the 18th century.

Marjorie M.K. Hlava, President, Access Innovations


Note: The above posting is one of a series based on a presentation, The Theory of Knowledge, given at the Data Harmony Users Group meeting in February of 2011. The presentation covered the theory of knowledge as it relates to search and taxonomies.