How did A.J. Ayer respond to skepticism in The Problem of Knowledge?

In his 1956 work The Problem of Knowledge, A.J. Ayer moved away from the aggressive reductionism of his earlier writings, adopting a more measured approach to skepticism.1

Rather than attempting to translate all propositions into sense-data, Ayer argued that constructions based on experience possess their own inherent validity. He addressed the radical skeptic—who highlights the gap between evidence and beliefs regarding the external world, other minds, and the past—by contending that such skepticism is largely unnecessary.12

Ayer maintained that if knowledge is defined by meaningful criteria that reflect how claims are actually justified and challenged in practice, then knowledge is accessible. He argued that skeptics often raise false problems, as there are established procedures for distinguishing between veridical and illusory experiences. Once these practical criteria are met, Ayer suggested that further doubt becomes meaningless. This shift marked a transition from his earlier, more rigid logical positivist stance toward a more subtle, moderate empiricism.23