Warning: I am highly biased, as Karl Popper is one of my most influential "mentors." As time passes, his wisdom increases, and his value as an original thinker becomes more, not less, vivid -- even if many of his "controversial" ideas in the Thirties are now considered normative.
Popper's collected essays, derived from class lectures, offers a broad introduction to the work of this seminal philosophy. These essays cover scientism, the scientific method, the scientific attitude, nominalism, historicism, democracy, falsification principle, evolutionary thought and applications, rationality, epistemology, and more.
While heralded as the scientists' philosopher of science, Popper's thought is not so provincial. His brevity and clarity of analysis are brought to bear on many subjects, practical and theoretical. His perspicacity and directness leave no room for ambiguity. The one philosophical topic not addressed in this representative volume is ethics.
Popper's central theme, of course, is science and how the scientific attitude and method fundamentally change our modern perceptions. While no longer controversial, indeed his thought has become commonsense, he, alone of the Vienna Circle, survived intact decades later. Because of the clarity, incisiveness, and rectitude of his claims, I purposefully return to him every five or so years to get "grounded" again.
One doesn't experience "eurkea" with Popper, one simply becomes reacquainted with basic knowledge and a few first principles. Perhaps a few "tweaks" occur, but Popper is more of an anchor than a revolutionary. Even his "defense" of science comes with numerous caveats. Given the topsy-turvy intellegensia stirring up the pot with new "-isms," it's useful to have a "home" to come back to. Because his commonsense prevails, his controversial stances several decades ago, while not quite platitudes now, are "defaults" that have withstood the strongest assaults. I cannot think of another major thinker who has withstood time and challenges better.
A couple of examples of Popper's gems: Democracy is not the best form of government; rather, it is the best form for excising bad government (this novel insight, a Popper first, is repeated by many subsequent political theorists, e.g., Ian Shapiro, Michael Walzer, John Rawls, etc.). An "open society" is more important, but this preeminent value requires the "background" of democracy. Central planning by governments should be confined to the margins, tinkering with changes that can be reversed before bad policy and unintended consequences become ensconced. If useful, then begin the reach. His skepticism does not permit purchase of any ideology. All historicisms are fortune-telling religious dogma, erroneously believing the past predicts the future, or that "inevitability" resides with the forces of History. Humans exist in an "open" environment, while science's predictability requires a "closed" environment; ergo, all "human sciences" are at best informed or educated guesses. Their ability to predict is next to nil.
Again, these Popperean gems may no longer be earth-shaking insights, but they once were, and the repitition of these claims is welcome against the ever-advancing onslaught of new "-isms." Popper's innately skeptical stances are a constant reminder that our fantasies can become our nightmares. This is most evident with science, where Popper insists that all knowledge, even scientific knowledge, is "tentative" at best. It's not just its verification, but ultimately its falsification, that requires this tentative stance. And, just because "science can," does not mean "science should:" Technology must "be harnassed."
An encounter with Popper leaves one speechless. Contentious by nature, I try to find loopholes in his claims; Popper does not leave many, if any. I'm still puzzled by his appeal to nominalism, but I cannot fault his logic. His thought experiment with tripartite worlds (not "universes") of the empirical, the conscious, and their overlap, is one of the best examples of Occam's Razor. But above all, Popper is as accessible as he is grounded. His clarity, brevity, and incisiveness are not common to philosophers, and thus, all the more welcome. He may not change your life, but he will provide a needed grounding for further venture!