As human beings, we all carry around with us a ‘little black box’ which physically we know as the amygdala and mentally the primitive stimulus-response mind. This box never turns off and constantly scans and monitors the environment for dangers.
Actually, it’s looking constantly for departures from an ideal — anything that looks odd, unusual, unknown, that might therefore be a threat or escalate into one.
It does so by continually projecting onto the environment an ideal scenario. The light of this ideal reveals the shadows of possible departures from it.
The ‘box' generates the light and beams it out across the physical and mental landscape and across time as well, projecting possible futures and ruminating over alternative pasts, perpetually seeking flaws, gaps, hollows, potential traps for the unwary.
Readers take this same box with them when they step into a fictional world. As they enter the partial trance which we call ‘reading’, they are also looking for departures from an ideal — clues given by the author that something is missing, something is not quite right, gaps, holes, oddities in sequence. These missing things create vacuums, by which readers’ attention is gripped immediately, just as the vacuums of real life trigger that little black box.
Hence the majority of protagonists in literature are orphans: the missing parents are the thing which instantly sucks in the attention of the reader. What other text books call the ‘inciting incident’ is in fact something in the plot which triggers the reader’s black box and draws them forward - and so on.
The same little black box keeps readers glued to the page with mysteries, embroiled in the plight of the protagonist with moral choices, and is then satisfied (or not) when the climax of the story resolves all these non-ideal situations and wraps the whole thing up — or leaves some situations intentionally open to give the reader goosebumps.
Reading is a parallel experience to living: in our lives, the little black box is part of our fundamental human condition and we are often guided by it to make choices on a day-to-day basis as well as longer term decisions, whereas in fiction we can go through the whole process vicariously and feel the thrills and rejoice in their resolution without the threat level impinging too much.
You can learn much more about vacuums in my book How Stories Really Work.