In what sense is it true that, had the past been in some way different, an aspect of the present would be more substantial?

Counterfactual history is a literary genre, not a mode of scientific argument, so it was a (pleasant) surprise to come upon the following in a thoughtful and provocative article entitled "Default positions: how neuroscience's historical legacy has hampered investigation of the resting mind" by F. Callard, J. Smallwood, and D.S. Margulies, which appears in Frontiers in Psychology:

"... a compelling body of literature – largely unknown or disregarded ... – ... documents various methods to investigate the processes characterizing activities pursued during so-called “idle time” ... what we now refer to as self-generated mental activity including daydreaming, fantasy, mind-wandering, and dissociation. It is not difficult to see that if these research domains had flourished ... rather than been neglected, our vocabulary for describing self-generated thought would have been richer; our methods to investigate such thought more creative; and hence our capacities to interpret the psychological meaning of different forms of spontaneous neural activity in different systems, including the mental life and associated neural systems of the [Default Mode Network], more substantial."