Logic Lens — Categorical Syllogism Analyzer

Why this project?

Categorical syllogisms — the cornerstone of formal logic since Aristotle — are typically taught through dry textbook exercises. Most online tools are simple multiple-choice quizzes. There's a gap for an interactive tool that lets you build syllogisms piece by piece, see the Venn diagram update in real time, and understand why an argument is valid or invalid.

What is it?

Logic Lens is a categorical syllogism analyzer. You choose three terms (major, middle, minor), select the proposition type for each premise and the conclusion (A/E/I/O), pick the figure, and the tool checks whether your syllogism is logically valid — showing animated Venn diagrams, the mood-and-figure classification, traditional names (Barbara, Celarent, etc.), and any fallacies committed.

How does it works?

Under the hood, Logic Lens implements the full validity checker for all 256 possible mood-and-figure combinations. It applies the five classical rules of syllogistic validity: (1) the middle term must be distributed at least once, (2) any term distributed in the conclusion must be distributed in its premise, (3) no two negative premises, (4) a negative premise requires a negative conclusion, and (5) two universal premises cannot yield a particular conclusion. The Venn diagram renders three overlapping circles with animated shading for exclusions and marks for inclusions.

How to use it

Choose terms: Click preset chips or type your own in the three term fields (Major = predicate of conclusion, Middle = connects both premises, Minor = subject of conclusion).
Select the Figure: Choose Figure I–IV to set the position of the middle term in the premises.
Pick proposition types: For the major premise, minor premise, and conclusion, select A (All), E (No), I (Some are), or O (Some are not).
Analyze: Click the green "Analyze Syllogism" button to see the verdict, mood, figure, traditional name, and any fallacies.
Randomize: Click the 🎲 button to generate a random syllogism and test your ability to spot valid ones.
Explore the Venn diagram: The animated diagram shows exactly how the premises interact — shaded regions mean "no members exist here," and marked regions (✕) mean "at least one member exists here."

Analyze Categorical Syllogisms