phiLean is an ambitious, open-source project to formalize philosophical arguments, concepts, and problems using the Lean 4 interactive theorem prover. The core library, philib, aims to be for philosophy what mathlib is for mathematics: a comprehensive, community-driven repository of formalized knowledge.

" whereof one cannot speak, thereof one must be silent. " — Ludwig Wittgenstein, Tractatus Logico-Philosophicus

In phiLean, we take this a step further: Whereof one can speak, one should speak precisely. If a philosophical problem cannot be stated in a well-typed, logically consistent form, perhaps the problem lies in the statement itself.

Many philosophical debates are plagued by linguistic ambiguity, hidden assumptions, and flawed reasoning. phiLean aims to address this by:

1. Clarifying, not just answering: Instead of trying to "solve" philosophical problems, phiLean provides a framework to dissect them. A poorly formed question often manifests as a type error or a logical contradiction under formalization.
2. Making Axioms Explicit: Philosophical disagreements often stem from unstated foundational beliefs. phiLean forces these into the open as explicit axioms, allowing for a clear analysis of how different starting points lead to different conclusions.
3. Enabling Computational Philosophy: By representing philosophical theories as code, we create a laboratory for exploring their logical consequences, comparing their consistency, and potentially even enabling AI to reason about complex ethical and metaphysical problems.

• Lean 4 installed via elan.
• A C compiler (like gcc) for the REPL.

1. Clone the repository:

   git clone https://github.com/imbue-bit/phiLean
   cd phiLean

2. Build the project to fetch dependencies (like mathlib):

   lake build

The easiest way to interact with philib is through our custom REPL. It's a lightweight C wrapper that starts Lean with the philib environment pre-loaded.

1. Compile the REPL:

   cd repl
   make

2. Run the REPL from the project root directory:

   ./repl/philean-repl

You will be greeted with a phiLean> prompt, ready to explore philosophy:

$ ./repl/philean-repl
philib environment loaded.
phiLean> #check Philib.Problems.Trolley.UtilitarianAnalysis.pull_lever_is_obligatory
Pilib.Problems.Trolley.UtilitarianAnalysis.pull_lever_is_obligatory : is_obligatory ...

phiLean> open Philib.Ethics.MetaTheories
phiLean> #check realist_excluded_middle
realist_excluded_middle : ∀ (theory : MoralRealism) (judgment : Philib.Ethics.Meta.MoralJudgment), ...

phiLean is a massive undertaking, and we welcome contributions from philosophers, logicians, and programmers alike. Whether you're a Lean expert or a philosophy student new to formal methods, there's a place for you here.

1. Start Small:
   • Add more theorems to existing files.
   • Improve documentation and comments.
   • Formalize a simple logical paradox in the Problems directory.
2. Tackle a New Concept:
   • Pick a concept from our planned extensions (e.g., Deontic Logic, Mereology).
   • Formalize a new philosophical thought experiment.
   • Implement a different ethical theory (e.g., Virtue Ethics).
3. Engage with the Community:
   • Open an issue to discuss a philosophical concept you'd like to formalize.
   • Review pull requests and provide feedback.

Our long-term vision is for phiLean to be a standard tool in the philosopher's toolkit. We imagine a future where:

• Philosophy papers include philib code snippets to verify their arguments.
• AI systems use philib to reason about ethical constraints.
• Students learn logic and philosophy through interactive, formalized examples.