Recollections of solid state physics mobility of individual electrons. This suggests that for very pure specimens the resistivity should be substantially less than that computed by Matthiessen's rule from less pure samples. This has now been verified, 4) so that the ideas of my 1931 paper have at last found some justification I have mentioned the electron-electron interaction, and this may be the place at which to refer to the question in Motts memorandum why this was so generally ignored. I have already referred to the classical argument that collisions between free electrons conserve momentum and hence the current. I realized that this argument was unconvincing, because there was in this respect a complete analogy between electron-electron and electron-phonon interactions. In the absence of Umklapp processes both conserve the wavevector (pseudomomentum). Neither conserves the electric current if the electrons are in Bloch states, but any conser vation law causes difficulty in restoring equilibrium, and hence in getting a finite resistivity. Both interactions can involve Umklapp processes, and then do not observe any conservation laws(except for energy) The original reason for neglecting electron interaction was that one was starting from simple approaches and generalized the model step by step, giving particular weight to those additional factors that one could see would make a major difference The effect of electron-electron interactions on the resistivity would lead, as I have mentioned, to a T 2 law, which had never been observed, and this suggested that th effect was probably small. This was borne out by a crude order-of-magnitude estimate I remember a conversation with Landau in which he explained how to estimate the contributions of various factors to the resistivity by dimensional reasoning This argument, which was more general than my rough estimate, showed indeed that the electron-electron collisions were negligible, except at temperatures below those in use at the time. He stressed that this was because in a degenerate fermi gas the only permitted transitions were those in which both electrons were initially, and remained, in the border region of the Fermi distribution, so that the 12 factor as really (kT/Er)2, whereas for the phonons the characteristic energy was the Debye cut-off ke. Evidently these ideas were the forerunners of his Fermi-liquid All of these discussions relate only to the effect of the electron-electron inter action on the irreversible processes, as distinct from its effect on the equilibrium state of the electron system. One guess, though not pursued in depth at the time, was that the Bloch wave functions and their energies might retain their meaning but that their explicit form would have to be modified (renormalized in modern terms I must confess that i never took too much interest in band calculations or in the lculation of the electron-phonon matrix elements(about which there was a separate, but not unrelated, controversy). I preferred to look at results which were not sensitive to these questions, partly out of laziness but partly out of pessimism about the possibility of making realistic band calculations, which would have t Vol. 37I. ARecollections of solid state physics 33 mobility of individual electrons. This suggests that for very pure specimens the resistivity should be substantially less than that computed by Matthiessen's rule from less pure samples. This has now been verified,(14) so that the ideas of my 1931 paper have at last found some justification. I have mentioned the electron-electron interaction, and this may be the place at which to refer to the question in Mott's memorandum why this was so generally ignored. I have already referred to the classical argument that collisions between free electrons conserve momentum and hence the current. I realized that this argument was unconvincing, because there was in this respect a complete analogy between electron-electron and electron-phonon interactions. In the absence of Umklapp processes both conserve the wavevector (pseudomomentum). Neither conserves the electric current if the electrons are in Bloch states, but any conser￾vation law causes difficulty in restoring equilibrium, and hence in getting a finite resistivity. Both interactions can involve Umklapp processes, and then do not observe any conservation laws (except for energy). The original reason for neglecting electron interaction was that one was starting from simple approaches and generalized the model step by step, giving particular weight to those additional factors that one could see would make a major difference. The effect of electron-electron interactions on the resistivity would lead, as I have mentioned, to a T 2 law, which had never been observed, and this suggested that the effect was probably small. This was borne out by a crude order-of-magnitude estimate. I remember a conversation with Landau in which he explained how to estimate the contributions of various factors to the resistivity by dimensional reasoning. This argument, which was more general than my rough estimate, showed indeed that the electron-electron collisions were negligible, except at temperatures below those in use at the time. He stressed that this was because in a degenerate Fermi gas the only permitted transitions were those in which both electrons were initially, and remained, in the border region of the Fermi distribution, so that the T2 factor was really (kT IER)2, whereas for the phonons the characteristic energy was the Debye cut-off kf9. Evidently these ideas were the forerunners of his Fermi-liquid theory. All of these discussions relate only to the effect of the electron-electron inter￾action on the irreversible processes, as distinct from its effect on the equilibrium state of the electron system. One guess, though not pursued in depth at the time, was that the Bloch wave functions and their energies might retain their meaning, but that their explicit form would have to be modified ('renormalized' in modern terms). I must confess that I never took too much interest in band calculations or in the calculation of the electron-phonon matrix elements (about which there was a separate, but not unrelated, controversy). I preferred to look at results which were not sensitive to these questions, partly out of laziness but partly out of pessimism about the possibility of making realistic band calculations, which would have to 2 Vol. 371. A