[en] Philippe Monthoux and the author have recently carried out detailed strong coupling (Eliashberg) calculations for YBa₂Cu₃O₇ of the influence of impurities on the normal state quasiparticle self-energy and resistivity, and the superconducting transition temperature, T_c. They find, in agreement with experiment, that although both Ni and Zn impurities primarily substitute for planar Cu(2) atoms, their influence on T_c is markedly different. That difference arises from the experimental fact that Zn, when substituted for Cu(2), alters the local magnetic order, and hence the local spin fluctuation spectrum, while Ni does not. This difference is the equivalent of the isotope effect for conventional superconductors: it is a smoking gun for the physical origin of high temperature superconductivity and provides strong evidence for the spin-fluctuation mechanism, and the d_x²-y² pairing state to which it gives rise