Here is one of two important passages in Book IV relevant to the Sachs difficulty. (By the way, N. White in his commentary says the difficulty was raised earlier by Grote, pp. 99-110, but I haven't been able to check that. Does anyone have Grote to hand?)
I've created an interlinear version using the text and translation from Perseus. I've put in bold a few words that struck me as especially noteworthy. Another text worth noting, as Sam Rickless points out, is 443e, which I'll post tomorrow.
This text reads--does it not?--as an induction, and in that respect it seems similar to many other Platonic texts where something is meant to be established by an examination of particular cases. Isn't this passage meant to be an argument, that internal justice implies ordinary justice? It appeals to our intuitive grasp of the 'power' of internal justice, of course; yet if one had to identify the genre of this passage, one would have to call it an 'argument' or 'proof'.
“Well then,” said I, “has our idea of justice in any way lost the edge of its contour so as to look like anything else than precisely what it showed itself to be in the state?” “I think not,” he said. [442e]
ti/ ou)=n; ei)=pon e)gw/: mh/ ph| h(mi=n a)pamblu/netai a)/llo ti dikaiosu/nh dokei=n ei)=nai* h)\ o(/per e)n th=| po/lei e)fa/nh;--ou)k e)/moige, e)/fh, dokei=.
“We might,” I said, “completely confirm your reply and our own conviction thus, if anything in our minds still disputes our definition--by applying commonplace and vulgar tests to it.” “What are these?”
w(=de ga/r, h)=n d' e)gw/, panta/pasin a)\n bebaiwsai/meqa ei)/ ti h(mw=n e)/ti e)n th=| yuxh=| a)mfisbhtei=, ta\ fortika\ au)tw=| prosfe/rontej.--poi=a dh/;
“For example, if an answer were demanded to the question concerning that city and the man whose birth and breeding was in harmony with it, whether we believe that such a man, entrusted with a deposit of gold or silver, would withhold it and embezzle it, who do you suppose would think that he would be more likely so to act [443a] than men of a different kind?” “No one would,” he said.
oi(=on ei) de/oi h(ma=j a)nomologei=sqai peri/ te e)kei/nhj th=j po/lewj kai\ tou= e)kei/nh| o(moi/wj pefuko/toj te kai\ teqramme/nou a)ndro/j, ei) dokei= a)\n parakataqh/khn* xrusi/ou* h)\ a)rguri/ou deca/menoj o( toiou=toj a)posterh=sai, ti/n' a)\n oi)/ei oi)hqh=nai tou=ton au)to\ dra=sai ma=llon h)\ o(/soi mh\ toiou=toi;--ou)de/n' a)/n, e)/fh.
“And would not he be far removed from sacrilege and theft and betrayal of comrades in private life or of the state in public?” “He would.”
ou)kou=n kai\ i(erosuliw=n* kai\ klopw=n kai\ prodosiw=n*, h)\ i)di/a| e(tai/rwn h)\ dhmosi/a| po/lewn, e)kto\j a)\n ou(=toj ei)/h;--e)kto/j.
“And, moreover, he would not be in any way faithless either in the keeping of his oaths or in other agreements.” “How could he?"
kai\ mh\n ou)d' o(pwstiou=n g' a)\n a)/pistoj* h)\ kata\ o(/rkouj* h)\ kata\ ta\j a)/llaj o(mologi/aj.--pw=j ga\r a)/n;
“Adultery, surely, and neglect of parents and of the due service of the gods would pertain to anyone rather than to such a man.” “To anyone indeed,” [443b] he said.
moixei=ai*/ ge mh\n kai\ gone/wn a)me/leiai kai\ qew=n a)qerapeusi/ai* panti\ a)/llw| ma=llon h)\ tw=| toiou/tw| prosh/kousi.--panti\ me/ntoi, e)/fh.
“And is not the cause of this to be found in the fact that each of the principles within him does its own work in the matter of ruling and being ruled?” “Yes, that and nothing else.”
ou)kou=n tou/twn pa/ntwn ai)/tion o(/ti au)tou= tw=n e)n au)tw=| e(/kaston ta\ au(tou= pra/ttei a)rxh=j te pe/ri kai\ tou= a)/rxesqai;--tou=to me\n ou)=n, kai\ ou)de\n a)/llo.
“Do you still, then, look for justice to be anything else than this potency which provides men and cities of this sort?” “No, by heaven,” he said, “I do not.”e)/ti ti ou)=n e(/teron zhtei=j dikaiosu/nhn ei)=nai h)\ tau/thn th\n du/namin h(\ tou\j toiou/touj a)/ndraj te pare/xetai kai\ po/leij;--ma\ Di/a, h)= d' o(/j, ou)k e)/gwge.