At their core, taxonomies and other classification systems are ways of organizing and managing knowledge. To understand the history of classification systems, it behooves us to explore the history of thinking about knowledge.

So what is knowledge? The Greek philosopher Plato defined it as “justified true belief”. However, the twentieth-century writer Bertrand Russell commented that “at first sight it might be thought that knowledge might be defined as belief which is in agreement with the facts. The trouble is that no one knows what a belief is, no one knows what a fact is, and no one knows what sort of agreement between them would make a belief true.”

Let’s back up to philosophy again, which is where we find the roots of theories of knowledge. Early philosophy encompassed several fields of knowledge, including some we’d expect (logic, metaphysics, theology, and ethics), as well as some we wouldn’t (physics, nature, and mathematics). Nowadays, philosophy encompasses (or is closely associated with) metaphysics, epistemology, ethics, aesthetics, and logic, while science, mathematics, and the like are their own domains.

Theory of knowledge began early. I’ve already mentioned Plato, who was born around 427 or 428 BC and died around 347 or 348 AD.









Plato’s stance was that knowledge of reality is what philosophy is all about. What we can take from this is that knowledge is all about understanding reality. Platonic realism connected knowledge with perception and observation of reality. The philosophy of realism was elaborated on by Saint Augustine (354-430 AD).










and by Saint Thomas Aquinas (1225-1274).











The thinking was that characteristics are what define objects (and, we might interject, classes of objects). Objects with different characteristics are not the same kind of object.

Then we come to William of Occam (or Ockham) (c. 1288 – c. 1348), an English friar and philosopher. His take on shared characteristics was a bit different.










Ockham rejected the shared universals as essences of some sort. He pioneered the metaphysical philosophy of nominalism, according to which shared types or characteristics can be represented by words.

As the Wikipedia article on nominalism explains:

“One wants to know in virtue of what are Fluffy and Kitzler both cats, and what makes the grass, the shirt, and Kermit green. The realist answer is that all the green things are green in virtue of the existence of a universal; a single abstract thing, in this case, that is a part of all the green things. With respect to the color of the grass, the shirt and Kermit, one of their parts is identical. In this respect, the three parts are literally one. Greenness is repeatable because there is one thing that manifests itself wherever there are green things.”

Felinity – represented by cats

* * Fluffy





* * Kitzler






Greenness – represented by green things

* * Grass





* * The green shirt





* * Kermit





As a conceptualist, Ockham ended up taking a stance between realism and nominalism. He maintained that universals exist, but as general concepts. They are dependent on our minds, and formed by extraction from particular experiences. So in the scheme below, we are the Knowers, and the concepts are the Known.







In the next installment, we’ll jump to the 17th 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.