"Every perception is to some degree an act of creation, and every act of memory is to some degree an act of imagination."
-- Gerald Edelman, Second Nature: Brain Science and Human Knowledge
1 On the role of cognitive science in the design of systems to augment cognition3 On the role of cognitive science in the design of systems to augment cognitionOn the role of cognitive science in the design of systems to augment cognitionAugmenting Long-term Memory2020-09-24Journal
Of course, there are many partial theories to answer these and other fundamental questions. But there's no single, quantitatively predictive, broadly accepted general theory. And so in that sense, we know little about distributed practice, and are probably decades (if not more) away from a reasonably full understanding.
To illustrate this point concretely, let me mention just one example: there are times when our memories don't decay, but get better over time, even when we're not aware of explicit acts of recall. Informally, you may have noticed this in your own life. The psychologist William James made the tongue-in-cheek observation, which he attributed to an unnamed German author, that\\ William James, “The Principles of Psychology” (1890).