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Introduction

What is Simplicity?

Delving Into Simplicity

What is Simplicity?

  • Simplicity in a nutshell
  • What can you do with Simplicity today?
  • What this course is — and isn't
  • Who is this for?
  • From articles to course
If you're coming to this course without a background in Simplicity, this chapter will orient you before we dive into the deep end.

Simplicity in a nutshell

Simplicity is a Bitcoin-native smart contract language, live on the Liquid Network today. First envisioned by Dr. Russell O'Connor around 2012 and detailed in his 2017 paper Simplicity: A New Language for Blockchains, it was activated on the Liquid Network in July 2025 after years of formal verification and development.
Unlike Ethereum's Solidity, which is a Turing-complete, high-level contract language, Simplicity is intentionally minimal. It has:
  • Three type formers (unit, sum, product)
  • Nine combinators (basic operations and composition rules)
  • No loops, no recursion, no dynamic memory
From just these primitives, you can build any computation you need for transaction validation, from boolean logic to full SHA-256 hashing.

What can you do with Simplicity today?

Simplicity is already powering real applications on the Liquid Network. The most notable is the Simplicity DEX, an oracle-free options marketplace where users trade call options on L-BTC using USDt as collateral (the underlying contract also supports puts). Other live Simplicity projects include Swaption by SideSwap (options) and the open-source Deadcat by Resolvr (prediction markets). Beyond DeFi, Simplicity enables advanced spending conditions such as vaults, covenants, and complex multisig schemes that would be impossible or unsafe in Bitcoin Script.

What this course is — and isn't

This is not a hands-on coding tutorial. You won't write Simplicity programs here. If you're looking for that, check out:
What this course is about: the philosophical and technical choices behind Simplicity's design. Why was this language created this way? Why only nine combinators? Why no recursion? Why does it matter that the type system connects to Gentzen's sequent calculus?
Think of it as understanding why the engine was built this way rather than learning to drive the car.

Who is this for?

This course is ideal for:
  • Protocol developers who want to understand Simplicity's foundations before writing code
  • Bitcoin researchers interested in the formal verification and type-theoretic approach
  • Computer scientists curious about the connection between sequent calculus and blockchain computation
  • Advanced bitcoiners who want to go beyond surface-level understanding of Liquid's scripting capabilities
If terms like "sum types", "combinators", or "sequent calculus" are entirely new to you, don't worry, we explain everything from scratch. But be prepared for a dense, mathematical journey.

From articles to course

The original "Delving Simplicity" series by Dr. O'Connor is structured as five technical articles. This course reorganizes and annotates that material into a progressive learning path with quizzes to test your understanding along the way. The ideas, definitions, and proofs are his, and we've adapted the format for structured education.