Scientists should be skeptics. We (if you will forgive me identifying as a scientist after a couple of decades away from any laboratory) should hold beliefs about the world lightly, being willing to give them up when contrary evidence turns up. Because of this, I sometimes hear scientists and rationalists say that we can't deny the existence of telepathy, or ghosts, or life after death, because physics might change in a century or two.

But is such skepticism really necessary? I started to question this after watching a great talk by the physicist Sean Carroll in which he explained that the physics of the human-scale world is complete. There are plenty of areas of physics where there is continuing mystery, but they are at scales of reality which have absolutely no consequence for our human existence. For example, the question of the nature of dark matter has nothing to do with biology. We don't notice the trillions of neutrinos that pass through our bodies each second, and dark matter is vastly less interacting with the world around us. And yet, I have, several times, come across people who say that dark matter might have something to do with consciousness or the soul! No, it definitely can't!

It seems to me that even though theories of how the world works can be refuted, this doesn't apply to the same degree to discoveries about what exists. We really aren't going to get evidence that DNA is not involved in human inheritance, or that electrons don't exist, or that the Earth is not orbiting the Sun.

So, do we apply the same degree of certainty to questions of mind and soul? The human-scale world has been explored in trillions of experiments and as a result we can talk of scientific certainty about principles of conservation of energy, principles which (it seems reasonable to say) render the existence of a soul physically impossible. At what point do we accept some ideas as facts?