For the Stoics, then, our judgments about the world are all that we can control, but also all that we need to control in order to be happy; tranquility results from replacing our irrational judgments with rational ones

And here lies the essential difference between Stoicism and the modern-day 'cult of optimism.' For the Stoics, the ideal state of mind was tranquility, not the excitable cheer that positive thinkers usually seem to mean when they use the word, 'happiness.' And tranquility was to be achieved not by strenuously chasing after enjoyable experiences, but by cultivating a kind of calm indifference towards one's circumstances.

Mainly, it’s not that there are things you can’t say. It’s that there are things you can’t say without the risk that people who previously lacked a voice might use their own freedom of speech to object.

Negative Capability, that is, when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason 

Sometimes the most valuable of all talents is to be able not to seek resolution; to notice the craving for completeness or certainty or comfort, and not to feel compelled to follow where it leads.

Ultimately, what defines the ‘cult of optimism’ and the culture of positive thinking – even in its most mystically tinged, New Age forms – is that it abhors a mystery. It seeks to make things certain, to make happiness permanent and final. And yet this kind of happiness – even if you do manage to achieve it – is shallow and unsatisfying. The greatest benefit of negative capability—the true power of negative thinking—is that it lets the mystery back in.