The rarest ability of all is to think honestly. Not to talk yourself into what you would like to believe, not to mistake a beautiful idea for the truth, not to take authority for proof. This has been taught in every age — and almost nobody listened. This route is about people who made honesty of thought the chief work of their lives.
Socrates never wrote a line. Everything we know of him comes through Plato. But his method has been alive for two and a half thousand years. He walked about Athens putting questions to men who considered themselves clever — and showed them that they did not understand the things they were talking about. "I know that I know nothing" is not modesty but an exact diagnosis. Honest thinking begins with the admission that you do not know.
Descartes resolved to doubt everything that can be called into doubt. The senses deceive. Memories deceive. Even mathematics — what if an evil demon is persuading us that two and two make four? One thing was left: I doubt, therefore I exist. Descartes' method is not skepticism for the sake of skepticism but an instrument: throw out everything that cannot be checked, and build knowledge only on what will stand.
Hume asked the question philosophy has not recovered from to this day: how do we know that the sun will rise tomorrow? Only because it has risen before. But the past is no guarantee of the future — it is simply habit. The whole of science rests on that habit. Hume was not saying that science is useless; he was saying: be honest about what it stands on. Do not pretend that we have rational answers to questions that are beyond our understanding.
Kant asked: what can reason know at all, and what lies beyond its bounds? His "Critique of Pure Reason" is a map with the borders drawn in: here is what we can know, and here begins what reason has no right to pronounce upon. God, freedom, immortality are not matters of knowledge but matters of faith. Honest thinking knows its own limits.
Pascal was a mathematician and a physicist — and a mystic at the same time. His "Pensées" is the most honest document there is of how thinking looks from the inside: inconsistent, self-contradictory, shot through with fear and hope. "The heart has its reasons, which reason knows nothing of" — this is not a renunciation of reason, it is the admission that a human being is more than a machine for drawing logical conclusions. Honesty requires admitting this too.
Feynman was no philosopher — he was a physicist. But his principle of honest thinking is more precise than many a philosophical system: if you cannot explain it in plain words, you have not understood it. If the facts contradict the theory, change the theory, not the facts. The most dangerous enemy of knowledge is the illusion of knowledge. In a world with ever more information and ever less understanding, this is the main thing to take in.
If honest thinking is this hard — why do intelligent people still do evil?