| As dictionaries explain, ontology is the philosophical study of the nature of being. It's one of the metaphysics, the branch of philosophy that tries to sort out the first principles, the ultimate roots of life, the universe and everything. What could be more basic than `being' itself? Indeed, ontology is also known as `first philosophy'.
Until very recently, the word ontology has been known to and used by a most miniscule fraction of humanity. It's one of those obscure, irrelevant words whose meaning even the highly educated tend to forget because they never have occasion to use it, except maybe when trying to make an erudite impression on the easily impressed.
Nevertheless, ontology is a term that has made the great leap from esoteric philosophy directly into engineering, bypassing the stage of science almost altogether. Now, ontology has become a buzzword in the fields of artificial intelligence, natural language processing, knowledge engineering and, of course, the World Wide Web. How did this happen? Let's start back at the beginning.
Traditional Philosophical Ontology
After millennia of deep thought by minds so bright that no ordinary person could look directly at them even while wearing Ray-Bans, ontology has divided into two main streams. One stream follows Plato and his lot in believing that realia, the things that actually exist, are beyond experience. That's idealism. The other stream follows Aristotle, Hume and a bunch of other more recent pro thinkers called the logical positivists. They tell us, not surprisingly, just the opposite: the objects of our experience are the things that actually exist. That's often called realism, of one kind. There are a few other isms that stick to ontology as well, solipsism, monism and dualism to name the main ones. They deal mostly with how many basic kinds of stuff exist and what that might mean.
It's Full of Objects
The idea of objects, their characterization and their possible interrelations within their existence has always been a part of ontology. In Platonic realism, the categories of things and properties of things have real existence as things in themselves and are based on a priori knowledge. Aristotle, on the other hand, held that some things exist only `in the understanding', or as mental phenomena that are built bottom-up from experience. Regardless of one's view of the nature of an object's existence, the shift in attention from the essence of being in itself to theorizing on objects and their properties, relationships and changes began to bleed ontology into epistemology, the philosophy of knowing. I would even say that this is where philosophy gives birth to what develops over the centuries into modern science, with it's emphasis on analysis and prediction as the basis of understanding.
The development of theories on the identification and definition of objects, their properties, their part-whole relationships, their class-member relationships, and other imaginable relationships has been the most active area in ontology in the last few centuries. Aside from perhaps the monist-dualist debate, whose life is gaining an extended lease from the encroachment of science deeper and deeper into the keeps of mentalism and spiritualism, ontology has become mainly a theorizing on objects.
Make It Reasonable
Ontology took a big step toward becoming something actually useful and meaningful outside the knitting circles of philosophy as a result of the influence of formal logic in the 19th century. Edmund Husserl seems to have been the first to call the marriage of formal logic and ontology `formal ontology'. The fruit of this marriage is that we can create formal systems that allow us to reason on the relations and properties of objects and to make assertions that are necessarily true about them based on our ontology. This serves as a knowledge multiplier.
For example, if our ontology tells us that there is a category of animals called quadrupeds that is defined by the feature of having four legs and some guy tells you that a `bulwargle' is a quadruped, then you know that a `bulwargle' is an animal that has four legs and shares all of the other defining features of animals and quadrupeds. "Wow!", you say. "Brilliant!" But you are being a bit of a sarcastic smartass. This is just `common sense', and you are not impressed.
What we need to remember, however, is that this is a formal system, which means that clever machines like your computer can use it. This is a way for your computer, or more exactly, a program that runs on your computer, to acquire and use `common sense' and to generate new knowledge on the basis of existing knowledge. This is the reason for all the buzz in fields such as natural language processing, knowledge engineering and artificial intelligence. The people who are engaged in designing and building practical systems in these areas are finally catching on to ways of efficiently representing knowledge structures that can be created, manipulated and shared by machines and used to interface with humans in a natural and useful manner. You might be more impressed now. You might be thinking of some cool ways an ontology could jazz up your favorite on-line community, like being the brain of an interactive function to teach you the ropes or help you actually find things.
- "What is an Ontology?", http://www-ksl.stanford.edu/kst/what-is-an-ontology.html
- "What is ontology? Definitions by leading philosophers", http://www.formalontology.it/section_4.htm
- http://www.w3.org/TR/owl-features/
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