Students are often told that E = F_q/q defines the electric field at a point if F_q is the measured force on a tiny charge q placed at that point. More careful instructors let q → 0 to avoid the polarization of nearby matter due to the presence of q. Unfortunately, this experiment is impossible to perform. A better definition uses F_q and the force F_−q measured when −q sits at the same point. There is no need to let q → 0, even if conductors or (linear) dielectric matter is present. Derive an expression that relates E to F_q and F_−q.