Re: [ontolog-forum] Distinction between ontology and semantics

[Thomas Johnston <tmj44p@xxxxxxx>]

A comment on understanding stuff, stuff like semantics and ontology.

Sometimes, when you're given a definition of a word, then you've "got it". You've got a pretty good idea of when to use the word and when not to. "Semantics" and "ontology" are not words like that. Knowing their definitions is almost worthless as a guide to when to use the words correctly and when not to, and provides you with almost no useful information, unless one can consider a list of relevant additional words to be useful information.

Having begun with that caveat, I offer these short definitions.

Semantics is the study of what makes words and sentences meaningful. A typical question in semantics is: "How do each of the words in a sentence contribute to the meaning of the sentence?" Another question is: "How does the syntax of a sentence contribute to the meaning of the sentence?" The biggest question in semantics is: "What is meaning?"

Ontology is the study of the basic categories of everything that exists. "Basic" usually means "most general". A simply ontology, then, might say that everything that exists is either a Thing, a Property of a Thing, or a Relationship among Things. The category Thing might be subdivided into Physical Thing and Abstract Thing. A critic might ask "What about Events and Processes? Don't they exist, too?" And the ensuing discussion will be a discussion about ontology. The biggest question in ontology is: "What is there?"

These "classical" kinds of ontologies are now called "upper-level ontologies". Their value is that they organize lower-level categories, and facilitate the consistent _expression_ of statements about those categories and about their relationships to other sets of lower-level categories. 

And so "mid-level" and "low-level" ontologies are also developed. A typical example of a mid-level ontology is an ontology for the transportation industry. A typical example of a low-level ontology is an ontology for LTL (less-than-truckload) transportation across a network of roads.

Formal ontology is the _expression_ of an ontology in a form where software can derive deductively valid statements from statements already explicitly made in the ontology. That is done by expressing the statements in an ontology in predicate logic form.

If a minimal set of statements can be identified, from which all others can be deductively derived, that minimal set constitutes the axioms of the system, and all derived statements are theorems. Software that can derive theorems from axioms is called an inference engine, and the process of an inference engine deriving a theorem (or determining that a given statement cannot be derived) is often undertaken in response to a query about the categories in an ontology.

The connection between semantics and ontology is that an ontology (unlike a dictionary) structures a set of definitions so that part of the meaning of the defined terms is expressed in connections among them that can be expressed in predicate logic, and that can be traversed by the said software. For example, an ontology for a business might contain the expressions "financial statement" and "invoice", and the statement "For all x, if x is an invoice, then x is a financial statement". This statement expresses the fact that, for that business, being a financial statement is part of the meaning of being an invoice.

I end by referring back to the caveat at the top of this comment.

Regards,

Tom Johnston

On Tuesday, July 21, 2015 11:14 AM, John F Sowa <sowa@xxxxxxxxxxx> wrote:

On 7/21/2015 9:50 AM, Phil Murray wrote:
> Dr. Obrst -
> you warn against conflating ontology and semantics, but...
> there certainly seems to be a lot of overlap.

Many terms overlap without being synonymous.

For a survey of the relationships, see the slides for a five-day
short course on "Patterns of Logic and Ontology":

    http://www.jfsowa.com/talks/patolog1.pdf

Slide 2 has the outline and URLs for the following four lectures.

Slide 4 has some very brief definitions of terms that are explained
in detail in the five lectures.

John

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