Method Of Philosophy

I am a bit torn. As I understand it, Plato is forming this city to determine if any act performed by man can be inherently good. To answer this question he forms this “perfect” city. I realize that this was established in the first chapter of this read, but something about it has continually bothered me. This method sounds great in theory, but I think in order to make it successful one would have to fully understand the human condition and in turn the deepest motives of man’s heart. Can anyone so accurately portray a scenario that one can draw a universal truth from it? Also how can one know that what is then found is truly a universal truth? Are there any universal truths when it comes to the motives of man?

~P.S. I commented on Caleb's post.