It belongs to epistemology, the branch of philosophy that considers the possibility, nature, and means of knowledge.
Since Gettier, "knowledge" is no longer widely accepted as meaning "justified true belief" only.
Klein concedes that, ultimately, the proper characterization of objectively available need be a member of this list, but, for the scope of Klein's defense of infinitism, he need not provide a fully developed account of objectively available reasons.
Objective availability could be best understood, at least as a working definition, as an existing, truth-apt reason not dependent on the subject.
"[3] It is simply an impossibility that a subject has an infinite chain of reasons which justify their beliefs because the human mind is finite.
"The infinitist is not claiming that in any finite period of time...we can consciously entertain an infinite number of thoughts.