Ever since the ancient Greeks conceived of the atom, humans have sought the smallest ingredients of our existence. In the past century, the use of reductionism to understand behavior has gained steam as the quantum universe and the workings of the human mind have been uncovered; still there are those who say that reductionists are oversimplifying our world. Here, acclaimed physicist Alastair 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? Rae exploration is an indispensable guide to one of the most fundamental ideas of science and the perfect companion for anyone considering how scientific findings reach into human life.