In this context, that means that we can (in principle) calculate the total electric field of many source charges by calculating the electric field of only q1 at position P, then calculate the fie...In this context, that means that we can (in principle) calculate the total electric field of many source charges by calculating the electric field of only q1 at position P, then calculate the field of q2 at P, while—and this is the crucial idea—ignoring the field of, and indeed even the existence of, q1.