One of the chief reasons for studying the past and reading old books, as for learning about our contemporaries in other cultures and other parts of the world, is to appreciate the tremendous diversity of human possibilities. It is, however, difficult to spend much time studying the past without being impressed by how similar people can be across wide spans of time and despite great differences of culture. For someone who, like me, has spent many years with his head crammed in books written over two millennia ago, 1859 AD doesn’t seem so long ago, and Victorian England doesn’t seem quite so different from America in 2017. But of course the differences are striking once we zoom in a bit; to take but a few examples, neither the lightbulb nor cocaine had yet been invented, women could not vote and the United States had about 4 million slaves, and probably nobody believed that it would ever be possible to create bombs that could kill millions of people in seconds. It was a different world. Yet John Stuart Mill could write this in On Liberty:

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