A bit of bad-assery from Thomas Hobbes:
Concerning the first, there is a saying much usurped of late, That Wisedome is acquired, not by reading of Books, but of Men. Consequently whereunto, those persons, that for the most part can give no other proof of being wise, take great delight to shew what they think they have read in men, by uncharitable censures of one another behind their backs.
But there is another saying not of late understood, by which they might learn truly to read one another, if they would take the pains; and that is, Nosce Teipsum, Read Thy Self: which was not meant, as it is now used, to countenance, either the barbarous state of men in power, towards their inferiors; or to encourage men of low degree, to a sawcie behaviour towards their betters; But to teach us, that for the similitude of the thoughts, and Passions of one man, to the thoughts, and Passions of another, whosoever looketh into himselfe, and considereth what he doth, when he does Think, Opine, Reason, Hope, Feare, &c, and upon what grounds; he shall thereby read and know, what are the thoughts, and Passions of all other men, upon the like occasions.
I say the similitude of Passions, which are the same in all men, Desire, Feare, Hope, &c; not the similitude or The Objects of the Passions, which are the things Desired, Feared, Hoped, &c: for these the constitution individuall, and particular education do so vary, and they are so easie to be kept from our knowledge, that the characters of mans heart, blotted and confounded as they are, with dissembling, lying, counterfeiting, and erroneous doctrines, are legible onely to him that searcheth hearts. And though by mens actions wee do discover their designee sometimes; yet to do it without comparing them with our own, and distinguishing all circumstances, by which the case may come to be altered, is to decypher without a key, and be for the most part deceived, by too much trust, or by too much diffidence; as he that reads, is himselfe a good or evill man.
But let one man read another by his actions never so perfectly, it serves him onely with his acquaintance, which are but few. He that is to govern a whole Nation, must read in himselfe, not this, or that particular man; but Man-kind; which though it be hard to do, harder than to learn any Language, or Science; yet, when I shall have set down my own reading orderly, and perspicuously, the pains left another, will be onely to consider, if he also find not the same in himselfe. For this kind of Doctrine, admitteth no other Demonstration.
If I'm understanding the last part of this section, I actually disagree. I agree with the caution about assuming everything of yourself, applies to everyone else, but I think Hobbes goes to far when he says this method--Read Thy Self--can only be applied to acquaintances.
"Read Thy Self" is the standard method I use to investigate slavery and the Civil War. At some point you tire of yelling about the evils of Nathan Bedford Forrest, and you settle into a much different frame. I believe, as Hobbes lays out here, that I am subject to the same whims as any slaveholder. I don't feel that there is anything in my bones that makes me any more moral. Thus the question becomes not "How awful was Robert E. Lee?" but "How could I have acted as he did?"
And you work to not ask that question with incredulity, but at the same time without apology It's "Read Thy Self" not "Construct Some Way To Excuse Thy Self." My favorite historians always manage this trick--explaining exactly how morality is violated, without endorsing the violation of morality. "Reasons" are not "excuses."