No one man ever completely understood the character of any other man.Or the way I say it, "People are complex." Godwin elaborates on this well; read the essay if interested.
A few sentences later Godwin writes:
Let every thing be examined, as far as circumstances will possibly admit, before it as assumed for true.Or in other words, figuring things out is hard.
I would go even a bit further than Godwin: if circumstances won't admit sufficiet investigation, then you don't know the truth. No excuse about unfortunate circumstances is good enough; the truth isn't easier to come by just because you have only limited ways of trying to find it.