I recently made this joke on Twitter:
There are of course plenty of non-corporeal entities which atheists ordinarily believe in, but many of them they have some alternative explanation for. Logical propositions are just expectations, as are mathematics, mathematical structures, etc. But they do tend to believe in the existence of agreements between people which are not easily reducible simply to expected behaviors.
In my tweet I chose one of the bigger agreements between people, the United States government, but really any sort of agreement will suffice, even something as simple as an agreement to borrow a shovel and return it when its use is finished. I like the example of the US government, though, since it exists not only through time but even through generations; it existed among people who are now dead and exists now among people who were not alive at its creation.
But clearly the United States government is not made up of any particular matter; there is no physical experiment one can device which would detect it. It has no weight, nor color, nor smell, nor sound. We know about it only through testimony and by its actions. And it only acts in the physical world through people.
In theory, if the principles that many atheists put forth were actually their principles, they would have to deny that the United States government exists, or at the very least claim that there’s no evidence for it.
But of course it’s not very hard to show that “their principles” are not in fact their principles. The much harder question is: so what are their principles?
That’s an excellent question I’m still trying to figure out the answer to.