How do you know?  He contends that the following “processes” yield knowledge:

-in the case of deductive truths: the ability to set out a valid deductive proof,

-in the case of inductive truths: these claims “…may be upheld by reference to perception, or to memory, or to testimony, or to historical records, or so scientific laws.  But such backing is not always strong enough for knowledge.  Whether it is so or not depends upon the circumstances of the particular case. 

It is not, however, easy to specify further what standards must be met by a putative knowledge claim.  Ayer contends that

9 ...to say that he knows is to concede to him the right to be sure, while to say that he is only guessing is to withhold it.  Whether we make this concession will depend upon the view which we take of his performance.  Normally we do not say that people know things unless they have followed one of the accredited routes to knowledge.” 

-10 Where there are recognized criteria for deciding when one has the right to be sure, anyone who insists that their being satisfied is still not enough for knowledge may be accused, for what the charge is worth, of misusing the verb `to know.’  But it is possible to find, or at any rate to devise, examples which are not covered in this respect by any established rule of usage.  Whether they are to count as instances of knowledge is then a question which we are left free to decide.”

-The sceptic who asserts that we do not know all that we think we know, or even perhaps that we do not strictly know anything at all, is not suggesting that we are mistaken when we conclude that the recognized criteria for knowing have been satisfied.  Nor is he primarily concerned with getting us to revise our usage of the verb `to know,’ any more than one who challenges our standards of value is trying to make us revise our usage of the word `good.’  The disagreement is about the application of the word, rather than its meaning.” 

The skeptics’ attack is directed toward the standards themselves. 

10-11 For Ayer, then, the necessary and sufficient conditions for knowing are:

that what one is said to know is true,

that one be sure of it, and

that one should have the right to be sure. 

10-11 For him, “this right 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, just as it would be a mistake to try to incorporate our actual standards of goodness into a definition of good.  And this being so, it turns out that the questions which philosophers raise about the possibility of knowledge are not all to be steeled by discovering what knowledge is.” 

Ayer contends that “The Quest for Certainty” has misled epistemologists:

11 it has been assumed that without a basis of certainty all our claims to knowledge must be suspect.” 

Sometimes `certainty’ is taken to mean `necessary’ or `a priori’.  This, of course, would rule meaningful empirical claims out of court completely! 

12 A priori statements can, indeed, be known not because they are necessary but because they are true and because we may be entitled to feel no doubt about their truth.  And the reason why we are entitled to feel no doubt about their truth may be that we can prove them, or even just that we can seem them to be valid; in either case there is an appeal to intuition, since we have at some point to claim to be able to see the validity of a proof....One is conceded the right to be sure when one is judged to have taken every reasonable step towards making sure; but this is still logically consistent with one’s being in error.  The discovery of the error refutes the claim to knowledge; but it does not prove that the claim was not, in the circumstances, legitimately made.” 

The issue here is whether the quest for certainty is a quest for knowledge or a quest for the exclusion of even the possibility of error.  That is:

certainty =?= knowledge (where one has the right to be sure)

or

certainty =?= knowledge (where one has no possibility of falling into error).  

As noted above, Ayer’s book endeavors to defend the former position.  His claim that we have standards which when properly applied provide legitimate claims to have the right to be sure (and, hence, legitimate knowledge claims), he does not need these need to be legitimated to overcome skepticism.  On p. 81 of his book he says:

81 “...we can give an account of the procedures that we actually follow.  But no justification of these procedures is necessary or possible.  One may be called upon to justify a particular conclusion, and then one can appeal to the appropriate evidence.  But no more in these cases than in the case of the more general problem of induction, can there be a proof that what we take to be good evidence really is so.  And if there cannot be a proof, it is not sensible to demand one.  The sceptic’s problems are insoluble because they are fictions.”