(Google) “the theory of knowledge, especially with regard to its methods, validity, and scope. Epistemology is the investigation of what distinguishes justified belief from opinion.”
(Merriam-Webster) “the study or a theory of the nature and grounds of knowledge especially with reference to its limits and validity.”
(Wikipedia) “( listen); from Greek, Modern ἐπιστήμη, epistēmē, meaning ‘knowledge’, and λόγος, logos, meaning ‘logical discourse’) is the branch of philosophy concerned with the theory of knowledge. Epistemology is the study of the nature of knowledge, justification, and the rationality of belief.”
Al was a scientist and yet also an original free-thinker so it is not surprising that he had his own understanding of knowledge and validity.