The error of leftist politics: there is no focus on the tools that help us to solve problems.
Instead, societies get boiled down to their mistakes. For example, the slave trade, imperialism, colonialism, etc. - all destructive, to be sure - are emphasized by leftists in favor of studying the successes of a society. How is this a mistake? Shouldn't we learn from the errors of the past?
Yes, but it is hardly a profound thing to point this out. It simply goes without saying. Much of history, in fact, is a conscious attempt to not repeat the errors of the past. What person is unfamiliar with the quote, "If you forget the past, you will be doomed to repeat it"? No one. Errors are striking, and noticeable. Perhaps they recall moments in childhood when we were punished for our errors. Our default is to pay attention to errors.
On the other hand, successes are far less likely than errors, and yet they go unstudied by amateur critics of society. This isn't surprising. Successes often arise from innovative thinking, or even paradigm shifts. This has the capacity to solve complex problems, and because problem-solving avoids error it can go unnoticed and under-appreciated. But innovative approaches to complex problems can allow us to navigate the world without resorting to fighting. After all, most people when faced with complex problems simply become frustrated and impatient - a precursor to violence. Innovative thinking can avoid this.
An example of one innovation: The right-left dichotomy of politics.
During the French Revolution, revolutionary leaders split the Parliament between those who were sitting "on the left" side and those sitting "on the right" side. This was a simple act, and probably did not seem important at the time. Yet this one simple act created a lens through which to view the world that proved useful for centuries! Henceforth, if you wanted to understand someone's political ideas, all you had to do was to run it through the left-right political dichotomy and you could easily conclude, "Ah, she's a right-winger," or "Ah, he's a left-winger."
To reiterate: this left-right dichotomy provided a lens through which to view all manner of complicated events and decisions. "So-and-so wants war in the Balkans" because he's a "right-wing hawk." Or "such-and-such policy" is "left-wing nonsense." World events were described in this way.
This was an imperfect but practical framework for navigating the complicated political landscape. It could be thought of as a function. You run some x-value (such as a political candidate's remarks during a debate) through this dichotomy and receive a corresponding y value ("he is left of center") which approximates reality. Or you could say it was a program through which you could run "political code" (such as social or fiscal policy), and allow the program to churn out a simple, concise explanation of the "political reality." Or you could just say the left-right political dichotomy was a heuristic of sorts - imprecise, but aiming towards a sense of what is true.
Learning From Success
But, as mentioned, this concept is imprecise. Reality is not a dichotomy. It is too complex. The left-right dichotomy is merely a useful concept that we project onto the world in order to give it a semblance of orderliness. As a concept, it allows people to make a bit of sense of an otherwise chaotic political landscape. But of course there is no doubt that this dichotomy also engendered much conflict and violence itself. People became polarized against left-right party lines, for example.
But again, we must learn not just from the error, but from the success. The error of political polarization must be heeded. The tribalism of the left-right dichotomy is obviously something to be avoided. And if one wanted to, he or she could focus on just this aspect of a useful concept. It is very easy, after all (even if necessary), to focus on the damage done by an idea or concept. It is perhaps more difficult to understand why such a concept worked, what utility it provided, and how it establishes a model we might follow to create other innovative ideas.