Once upon a time, the nature of the intellect was understood (by Socrates, Plato, Aristotle, Aquinas, et al.) to seek the truth, that unchanging reality that is true for all of us (thus, a “common truth” that guides the “common good”).
Alas, the Enlightenment severed the intellect from natural law, or from any grounding in an unchanging truth. So today’s “rationalist” is really a relativist: he uses his intellect, to be sure, but not to serve or even seek the unchanging logos of Heraclitus; he does not even acknowledge the existence of truth. Instead, it gives us “what’s true for you isn’t true for me” as the guiding principle of undergraduate life — an attitude that, these days, lasts a life-long. So everybody puts his intellect to work serving his own “truth” — even if that truth is merely a base appetite for another free lunch, which for the purpose serves as the “highest good.”
A sidebar: This also helps to explain why folks nowadays tell you how they “feel” about the Fed, or Congress, or even liberty: you can’t argue with a feeling. If they told you how they "think," you could have a rational (in this case, reasonable) discussion. You can argue with a rational opinion, if it is articulated as such. But a lot of folks intentionally evade argument — they want to be left in the comfort zone of their own irrational meanderings (down there in Plato’s Cave). So they convey their opinions as feelings. Argue, and they sternly object: “You can’t tell me how I feel!”
Historically, this sundering of the intellect from truth produced Hobbes’s “Mortal god.” The language of the modern Leviathan is the dialectic. For the doomed subject, it’s DoubleThink.
Solzhenitsyn put it his way: The truth will make you free, but falsehood always brings violence in its wake.