Expert System
Last Updated: July 29, 2026 | By Mihail Sebastian | AI Dictionary
Software that captures a human specialist's knowledge as explicit if-then rules and applies them to give advice in one narrow domain, such as medical diagnosis.
What is an Expert System?
An expert system is software that captures a human specialist’s knowledge as explicit if-then rules, then applies those rules to give advice in one narrow domain. Expert systems were the dominant form of commercial AI in the 1970s and 1980s, before machine learning displaced hand-written rules as the standard way to build intelligent software.
The term still earns its dictionary entry: it names the approach (encode what experts know) that modern AI inverted (learn from data what no one wrote down).
How Expert Systems Work
An expert system has two parts. The knowledge base holds the rules, gathered by knowledge engineers who interview specialists and translate their judgment into statements like “IF the patient has a fever AND the culture is gram-positive, THEN consider these organisms.” The inference engine chains rules together, asking the user for missing facts until it reaches a conclusion.
Because every conclusion traces back to named rules, an expert system can explain its reasoning step by step. That built-in interpretability is something modern neural networks struggle to match.
The approach faded for practical reasons. Extracting rules from experts is slow, expensive work (the “knowledge acquisition bottleneck”), and finished systems failed abruptly on cases just outside their rules.
Example of an Expert System
MYCIN, built at Stanford University in the 1970s, is the canonical case. It questioned a physician about a patient’s symptoms and lab results, applied several hundred if-then rules, and produced a ranked list of bacteria likely to be causing an infection, with recommended antibiotics and doses.
Research evaluations at the time found its advice compared well with that of infectious-disease specialists. MYCIN never entered routine clinical use, but it demonstrated that encoded expertise could work and shaped how a generation of AI systems was built.
Related AI terms: Weak AI · Turing Test · Knowledge Representation · Fuzzy Logic
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Mihail Sebastian — Writes about AI governance, regulation, and the technology behind them. Placeholder bio — replace with a real credential line. About