Within about a century their deep thoughts will spread to virtually every mind, where they will clarify or distort every conclusion, and shift every value-judgment toward the better or the worse. Philosophy drives the course of history, very often in directions which would have astonished a philosophy's first adherents. E.g., skepticism has long been thought of as a stick with which to beat religion. So most men would be baffled to learn that David Hume's 18th century skepticism led to a 20th century eruption of Eastern mysticism in the West. Hume to Kant, to Hegel, to Schopenhauer, to a bunch of lesser lights, to 1960s hippiedom.
History is the laboratory notebook recording man's philosophical experiments.
Just as, in the special sciences, one experiment can be worth a library full of theorizing, so it is in philosophy. Becoming a deep thinker requires innumerable judgment calls. This precedes the tidy arguments which package what you learn in a form that you can build on, and that others can learn from you. And your only guide in making these judgment calls is your own honesty. It also takes an enormous amount of reading, and quite a long time.