Now, in a world in which Fido lives solely to go on walks, leash becomes the most important thing. If Fido can find it reliably on a certain table or inside a certain closet, he may ignore large portions of his domestic reality.
People in the thrall of romantic infatuation also experience this magical effect. No other prospective mate has meaning. The attentions of those candidates may be completely ignored. To those rejected suitors, it may seem as though they don't exist at all to the subject of their intentions.
Where writ here in the small, so has been the course of human understanding. Primitive men lived in an extremely rich perceptual environment. In order to understand that environment, they had to narrow their perception of events. Objects successfully categorized became the focus of their perception, excluding the apprehension of phenomena that appeared less relevant to accomplishment of their goals.
In other words, living creatures, whether through Darwinian or Lamarckian processes, control their perceptions in order to improve their control of outcomes. With regards to human intellectual achievement, understanding has a cost in experience: our perceptions deepen, but narrow.
I have already observed that thought has allowed us to dramatically change the nature of the reality we inhabit. Possibilities and outcomes are far different now than they were when mankind first began the experiment of understanding. It may be impossible for us to apprehend exactly how different their perception of reality was from ours. I have found it instructive, however, to consider "Why?"