The atomic theory of Leucippus and Democritus represents the culmination of two centuries of investigation into the conceptual modelling of ultimate reality. From a historical perspective, the theory encapsulates the ideas of thinkers ranging from Thales, Anaximander, and Heraclitus to Pythagoras, Parmenides, Zeno, and Empedocles, as well as Gorgias, Protagoras, and Anaxagoras. From a didactic perspective, Leucippus and Democritus demonstrate meta-modelling and transcendental modelling techniques that will influence philosophy, science, and religion up to our days. From the relation between mind and body to the interaction between the soul and the senses, the atomic theory provides a consistent framework for the understanding of ultimate reality.