Full Idea
Phenomenalism offered the prospect of explaining perceptual knowledge within a framework that recognised only conclusive reasons; once it is acknowledged that at least induction uses nonconclusive reasons, it is no longer needed.
Gist of Idea
Phenomenalism offered conclusive perceptual knowledge, but conclusive reasons no longer seem essential
Source
J Pollock / J Cruz (Contemporary theories of Knowledge (2nd) [1999], §2.3.3.2)
Book Reference
Pollock,J.L./Cruz,J: 'Contemporary Theories of Knowledge (2nd)' [Rowman and Littlefield 1999], p.40
A Reaction
I'm not sure that that is the only motivation for phenomenalism, which seemed to be attempting to get as close to 'reality' as intellectual honesty would allow. I certainly favour the modern relaxed attitude to knowledge, which needn't be 'conclusive'.