What's an Ontology? (And Why Should You Care?)
Everyone's suddenly talking about “context graphs” and “semantic layers.” Underneath all that hype? An idea from the 1990s called an ontology.
The Confusion Problem
You're in a meeting. Someone says “we need better knowledge management.” Another person mentions “context graphs.” A third drops “semantic layer.” A vendor promises “ontology-driven AI.”
Are these the same thing? Different? Complementary?
Nobody in the room is quite sure. But everyone nods. Let's fix that.
Start With What You Already Know: The Taxonomy
A taxonomy is a classification system. It puts things into boxes.
Example: The Library
- 600s → Technology
- 610s → Medicine
- 616 → Diseases
- 616.1 → Cardiovascular diseases
Clean. Hierarchical. A tree structure where everything has one parent.
In healthcare, ICD codes work the same way:
- Chapter IX — Diseases of the circulatory system
- I10-I16 — Hypertensive diseases
- I10 — Essential hypertension
Taxonomies answer one question: “What category does this belong to?”
Now Add Relationships: The Ontology
An ontology takes the taxonomy and adds connections between concepts.
Back to the library. The Dewey Decimal System tells you where the book lives on the shelf. But a library catalog is richer:
- → This book is written by Author X
- → This author also wrote these other books
- → This book is related to that book
- → This edition supersedes the previous edition
- → This book is required reading for Course Y
Same books. But now you can navigate between them in meaningful ways.
In Healthcare: SNOMED CT
It doesn't just classify “Type 2 Diabetes” — it captures:
Metabolic disease
Metformin, Insulin
Hypertension, Obesity
HbA1c test
Ontologies answer: “How is this thing connected to other things?”
The E-Commerce Example
You shop on Amazon. Here's taxonomy vs. ontology in action:
Taxonomy (Category Tree)
Electronics
└─ Computers
└─ Laptops
└─ Gaming Laptops
Helps you browse.
Ontology (Relationship Graph)
- →This laptop is compatible with these accessories
- →Customers who bought this also bought that keyboard
- →This model is the successor to last year's model
Makes recommendations actually useful.
Why This Matters for AI
Large Language Models (LLMs) like GPT-4 or Gemini are trained on text. They learn patterns in language. But they don't inherently understand relationships between concepts in your specific domain.
Without Ontology
“Can a patient on Warfarin take Aspirin?”
The LLM guesses based on probability patterns. Maybe it gets it right. Maybe it hallucinates.
With Ontology
- Fact: Warfarin is_a Blood Thinner
- Fact: Aspirin is_a Blood Thinner
- Rule: Blood Thinner contraindicates Blood Thinner
The ontology gives the AI structured reasoning paths.