This special double issue of Cybernetics and Human Knowing is comprised of a collection of papers devoted to the cybernetics and mathematics of Charles Sanders Peirce with a special focus on its synergies with George Spencer-Brown's thinking.Peirce was a truly original American philosopher and logician working in the late 1800s and early 1900s; Spencer-Brown is an English polymath, best known as the author of Laws of Form.The contributions reflect the extraordinary richness of Peirce's work and his relevance to present concerns in cybernetics.The similarities in the focus on some of the deep foundational subjects are astonishing, amongst those especially the concept of the void or Firstness and the continuity of mind and matter.