Ever since the ancient Greeks conceived of the atom, humans have sought the smallest ingredients of existence. In the past century, the use of reductionism to understand behaviour has gained momentum as the quantum universe and the workings of the human mind have been uncovered in particle colliders and fMRI scanners. Acclaimed physicist Alastair I.M. Rae spells out how the powerful tool of reductionism works, from the level of subatomic particles, up through molecular chemistry, and beyond to our neural networks. How does physics explain consciousness? Can quantum mechanics be applied to the brain or mind? What can economists learn from reductionism? Rae's exploration is an indispensable guide to one of the most fundamental ideas of science.