There exists a tendency in nature to reduce complexity via modularization. This tendency grows when more modules become available. Finally this tendency enables nature to create intelligent and very sophisticated creatures.
I encountered relations in several areas of physics and in human interactions.
Physics is based on relations. Quantum logic is a set of axioms that restrict the relations that exist between quantum logical propositions. Via its isomorphism with Hilbert spaces quantum logic forms a fundament for quantum physics. However, quantum logic only describes static relations. Classical logic is a similar set of restrictions that define how we can communicate logically.