The true sting here isn't in the first half—we rather like the idea of thinking independently—but in the second. Voltaire is asking us to tolerate, even protect, the right of people we disagree with to reach conclusions we find wrong. That's vastly harder than mere intellectual freedom; it's a plea for restraint against the very certainty our own thinking produces. When your teenager rejects your religion, or your neighbor votes contrary to your values, this quote asks you to resist the urge to correct, cajole, or convert—to let their reasoning process work itself out, however it may. The privilege he names isn't abstract; it's the mundane, daily choice to bite your tongue and accept that someone you love might arrive at a different truth.