I haven’t read this yet, but my take on “Gettier” problems is that they break on the issue of “justified” rather than “belief”. (And further that the break is so obvious that the seriousness with which they are discussed undermines my respect for the discipline of academic philosophy.) Basically, that the justification required to qualify a belief as knowledge is a lot stronger than that required to protect the believer from censure as intellectually irresponsible. Will read further and maybe comment more later.
In fact, I do think that the definition of “justified” is the crux of the problem of knowledge, and I agree with Ayer’s claim that it “is to be decided, if at all, on grounds of practical convenience”. The right to be sure “may be earned in various ways; but even if one could give a complete description of them it would be a mistake to try to build it into the definition of knowledge”. Ayer devotes a good part of his book to repeatedly emphasizing this point – that what he is concerned with is not about “discovering what knowledge is” but rather about when it is reasonable to claim it (or attribute it to others) even though that claim (or attribution) may actually be wrong. All this in the very book that Gettier cites as tying Ayer to the JTB “definition”. Did he ever actually read it? Perhaps not (unless we wish to accuse Gettier of plagiarism), since it was in that same book seven years before Gettier that Ayer also wrote “If a witness is unreliable, his unsupported evidence may not enable us to know that what he says is true, even in a case where we completely trust him and he is not in fact deceiving us” – which includes most Gettier examples if one has the wit to interpret “unreliable” witness as including cases where the person doing the witnessing is unaware of some essential aspect of the context.
All this is not to deny the value of Gettier examples as a paedagogical tool for explaining the difference between a morally justified and a logically justified opinion, and that the latter may only be strictly achievable in very limited contexts (such as pure mathematics) – if at all.