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Ricard Solé (1962–)

Solé’s work on complex adaptive systems maps the territory where cognition emerges without central control. His liquid brains — ant colonies, immune systems, microbiomes — are decentralised systems where intelligence arises through transient interactions between mobile agents, with no fixed wiring and no hierarchy imposed from above. His criticality work shows living systems operating at the boundary between order and disorder — the fertile ground where complexity grows. And his cognitive morphospace, mapping all possible architectures with large regions unexplored, reveals structural possibilities waiting to be walked. SPLectrum reads its own account of decentralised, relational complexity in Solé’s scientific territory.

Ricard Solé (1962–). Complex systems scientist, ICREA Research Professor at Universitat Pompeu Fabra (Barcelona) and External Professor at the Santa Fe Institute (since 1997). Dual five-year degrees in physics and biology from the University of Barcelona; PhD in physics from the Universitat Politècnica de Catalunya — a rare combination that explains why his work bridges statistical mechanics and living systems. His entry into complexity science came through extinction dynamics: showing (with Susanna Manrubia, 1996) that a simple model of co-evolution produces power-law distributed extinction events — contributing to the debate on whether mass extinctions are self-organised critical phenomena. His overarching programme investigates general laws underlying the emergence of complex adaptive phenomena, from viral evolution and ecosystem collapse to the origins of cognition.

Key concepts

Liquid brains and solid brains. A framework for mapping cognitive architectures. Solid brains have fixed wiring — persistent neurons, stable synaptic connections, memory stored in connection weights. The vertebrate nervous system is the canonical example. Liquid brains have no permanent connections — mobile agents interact through transient encounters, and the network topology is constantly rewired by physical movement. Ant colonies, slime moulds, immune systems, and microbiomes are liquid brains. Memory in liquid systems is carried not in connections but in population densities and chemical gradients. Published in “Liquid brains, solid brains” (Philosophical Transactions of the Royal Society B, 2019, with Melanie Moses and Stephanie Forrest).

The cognitive morphospace. A multi-dimensional space mapping all possible cognitive architectures — defined by axes such as whether components are mobile or static, whether they contain neurons, and computational complexity. Known biological and artificial systems leave large regions empty. Some voids may be physically forbidden; others are simply unexplored. Solé’s synthetic biology programme aims to probe whether some can be filled artificially.

Criticality and phase transitions. The idea that living systems operate at or near critical points — the boundary between order and disorder — traces back to Chris Langton and Stuart Kauffman in the early 1990s. Solé’s contribution is demonstrating it empirically and formally in specific systems. In ant colonies, collective cognition emerges at a critical density through a percolation-like phase transition — below that density, no colony-level intelligence; above it, adaptive collective behaviour appears. His book Phase Transitions (Princeton, 2011) shows how the same mathematical structures — critical points, symmetry breaking, universality — appear across gene networks, epidemics, language evolution, ecosystem collapse, and the origin of life, in systems that share no material substrate. At criticality, systems achieve maximum information storage, fast response, and computational flexibility.

Synthetic major transitions. Using synthetic biology to recreate key evolutionary innovations in the lab — protocells, multicellularity, symbiosis, cognition. The aim is to understand which transitions are likely, which are contingent, and what minimal conditions enable them.

Key works

“Extinction and self-organized criticality in a model of large-scale evolution” (Physical Review E, 1996, with Susanna Manrubia) — power-law extinctions, the entry into complexity science
Signs of Life: How Complexity Pervades Biology (Basic Books, 2000, with Brian Goodwin) — complexity in biological systems
Phase Transitions (Princeton University Press, 2011) — critical phenomena across complex systems
Viruses as Complex Adaptive Systems (Princeton, 2012, with Santiago Elena) — viral evolution through a CAS lens
“Liquid brains, solid brains” (Phil. Trans. R. Soc. B 374, 2019, with Moses & Forrest) — the framework for distributed cognitive architectures
“Statistical physics of liquid brains” (Phil. Trans. R. Soc. B 374, 2019, with Jordi Piñero) — formal models: Hopfield networks, percolation, criticality in liquid systems

Where Solé stops

Solé maps the architecture of cognition — but he stays with the physical and the computational. How liquid brains constitute shared meaning, how the transient interactions carry language in SPLectrum’s broad sense, how the emergent intelligence of an ant colony relates to the intelligence of a neural network as interrelating languages — these are not his questions. His systems process information; SPLectrum’s languages constitute reality. The structural parallel is real — decentralised, no central controller, complexity emerging at the edge — but SPLectrum asks what happens when the medium is not just information but meaning.