Scholars in ancient philosophy need to stop speaking of 'the Sachs difficulty' and begin speaking, rather, of 'the Grote difficulty'.
Here is Penner's characterization of Sachs' objection, from the first page of his recent paper:
In David Sachs’s classic 1963 article ‘A Fallacy in Plato’s Republic,’ Sachs finds Plato guilty of what he calls a ‘fallacy of irrelevance’ in the response he has Socrates give to the Thrasymachean challenge. What this fallacy consists of, in brief, is (as I shall put it) Socrates’ ‘changing the subject’ on Thrasymachus, Glaucon, and Adeimantus. Their challenge is a challenge to justice on the ordinary conception of justice, which Sachs sees as consisting in ‘the non-performance of acts of certain kinds.’ (Though he does not say so explicitly, it is clear from other remarks in Sachs that he thinks of this nonperformance conception of justice as identical with the ordinary moral conception of justice. Sachs calls this the ‘vulgar’ conception of justice, as though it is the conception which most ordinary people work with). But what Socrates does in response is to introduce a new conception of justice - one which would not have occurred to Thrasymachus, Glaucon, or Adeimantus in a million years unless Socrates had proposed it, and which probably would not even have occurred to us if we had not read the Republic. Socrates then argues - employing that new conception of justice - that the just are always happier than the unjust.But Grote had said the same thing, that Plato is guilty of equivocation ('changing the subject', the 'fallacy of irrelevance'):
Now in regard to the definition here given by Plato of Justice, we may first remark that it is altogether peculiar to Plato; and that if we reason about Justice in the Platonic sense, we must take care not to affirm of it predicates which might be true in a more usual acceptation of the word...These are surely the same objection.
The narrower sense is that which is in more common use; and it is that which Plato assumes provisionally when he puts forward the case of opponents in the speeches of Glaukon and Adeimantus. But when he comes to set forth his own explanation, and to draw up his own case, we see that he uses the term justice in its larger sense...
He appears to be successful only because he changes the terminology, and the state of the question...
In his footnote 67, however, in which he refers to Grote, and argues that Sachs has a different criticism, Penner refers to other passages in Grote, which are not directly to the point (committing the fallacy of irrelevance, as it were). Read Penner and say whether he has represented Grote's complaint well:
Grote 1888, ch. 34, also speaks in terms of a Platonic sense vs. an ordinary sense. But the gravamen of his charge is not so much the new Sachsian charge that Plato commits a fallacy in reasoning, as the old charge that Plato’s account of the reference of ‘justice’ is wrong. Justice cannot be what the Republic says it is - a single internal ‘entirely self-regarding’ state (‘a state of internal happiness to the agent’). What it has to be, Grote insists, is rather a state of exchange - ‘reciprocity’ - between its possessor and others, in which the possessor exchanges the fulfilling of his or her obligations to others for the receiving of his or her rights from others. (In just this way, Grote will have thought the Greatest Happiness of the Greatest Number will have been served -- the true idea of justice, in Grote’s view). This should, indeed, have been the upshot, Grote says, of taking seriously the idea of justice being based naturally on our need that each ‘does his own’ (369bc with 370c) -- presumably since each needs that the other does his own. It should also have led Socrates to grant that fathers do act correctly in praising justice to their sons for its consequences in how other people see us. Socrates is quite wrong to suggest (368c) that it would be a slander on justice to allow such consequences to be the real basis for justice being a virtue. (Grote usefully cites Laws 950b as an implicit criticism of Republic Book II).By the way, Grote introduces his point about reciprocity in order to argue, not merely that the Platonic notion of justice is different from the ordinary notion, but also that the two notions cannot be bridged, because the ordinary notion essentially involves reciprocity, whereas the Platonic notion does not.