How does influence travel from one thing to another? In Newton’s mechanics of particles and forces, influences travel instantaneously across arbitrarily far distances. Newton himself felt this to be incorrect, but he did not suggest a solution to this problem of “action at a distance.” To solve this problem, we need a richer ontology: The world is made not only of particles, but also of fields. As examples of the field concept, we study the theory and applications of the electric field and the magnetic field. Students will learn how fields are generated, how fields interact with matter and with each other, and how these interactions inform our understanding the world.