The men who achieved the greatest advances in the mental exploration of space were of many kinds, but they had two characteristics in common: doubt and faith. They doubted enough to keep their minds from being blocked by cherished ideas that might be antithetical to what they were trying to discover. They believed with all their might that the universe was subject to laws of perfect order, and that any appearances of caprice could be resolved. They worked hard to achieve this, struggling with their own prejudices as well as with other people’s old ideas and incorrect observations. And they emerged triumphant, reaping rewards greater than any of the creature comforts that they denied themselves for the sake of their cause. They discovered planets.