"Twenty-five centuries ago, Plato taught this doctrine, but even the educated nowadays find it difficult to understand."

Difficult to understand..? Yet is isn't. It's actually fairly cut & dried. I think the "difficulty" enters into the picture as mans' ego & sinful nature eclipses the perceived need for a structure, moral or otherwise, that impinges upon his need to do as he pleases whilst he ekes out his existence on terra-firma. As sin is piled upon sin ad infinitum, the "progressive" man reasons that, in order to remain progressive, he needs only keep walking along his present course. Sure things seem to be "getting bad", but, after all, he hasn't murdered anyone (least not by his own literal hand). So, in the name of progress, he slogs on, unsurprisingly & increasingly unaffected by the world that is formulating around him. Were he to come to his senses, he'd realize that true progressiveness would be to turn around one-hundred-and-eighty degrees and begin walking back towards his original starting point: the crossroads of truth and morality. But that would run counter to the primordial "instincts" he's so wont to tout. I mean...why choose cold, hard morals when all is comfy-cozy under the warm, inviting blanket of sin?