Perfectly repeatable and connectable remembered-thoughts establish the means for perceiving causal relationships, more so than any remembered-external-event, and therefore causation most likely roots in remembered-thinking.
More importantly, Hume accomplished the difficult philosophical task of describing the human mind, from an epistemological standpoint, as an entity that begins learning about the universe without any pre-set knowledge or capacity to learn about the universe, aside from what static snapshots the senses provide.
In the second sentence, Einstein reports on an experience that must be described as remembering-thoughts-remembered-about-remembering-remembering, or some immense layering of complexity in remembered-thinking. There are reports of the memories, which he remembers-remembering, and reports of feelings, that he remembers feeling in association with the remembered-remembering. But most importantly, there is in the sentence an explicit analysis about the connection between different units of remembered-remembering, guided by remembered-feeling.
By darkness I mean the space behind or around the memory of a thought. If you wanted to explore more deeply the memory of the show at a music festival, you may return to the scene afterwards to get a better understanding of the geography, or attend the festival again next year to fill in some contextual gaps, or see the band again, meet them and ask about their perceptions of the same event. This investigative possibility does not exist for remembered-thinking, all a person has is introspection. This is what I mean by a deep dive into the darkness.