Is metaphysics just a bunch of nonsense? Is it okay to believe something you could never prove? Could logic be a solution to the world’s problems? This week on Philosophy Talk, we’re thinking about the Vienna Circle, a group of Austrian philosophers from the 1920s who debated these questions in an attempt to usher in a new era of scientifically grounded thinking.

While the philosophers of the Vienna Circle (including Rudolf Carnap, Otto Neurath, and the circle’s central figure, Moritz Schlick) had plenty of disagreements, they agreed on some general principles: science and logic are the best tools for understanding the world; a statement is only meaningful if you can test it using experiments and observations; and metaphysics is meaningless.  

Many religious debates are meaningless by the standards of the Vienna Circle. Sure, it seems like everyone understands what you’re saying when you talk about God, but the philosophers of the Vienna Circle would disagree. God’s existence makes no difference to what we can see and hear and touch. The fact that we can’t disprove Her existence, any more than we can disprove the existence of the Loch Ness Monster, is no help at all.

But what exactly does it take to test a statement against observation? There are statements that you can’t prove even if they’re true, but can still disprove if they’re false. (For example, observing one black swan is enough to disprove the theory that all swans are white, but no amount of observing white swans is enough to prove it… what if there’s a black swan somewhere that you’ve missed?) Is a theory disproved by an observation that it counts as extremely improbable, but not impossible? And if your lab experiments contradict the accepted laws of physics, shouldn’t you just conclude that your equipment is broken?

Mathematics also creates trouble for the idea that all meaningful statements can be tested experimentally. We know that 2+2=4, and it seems like no experiment could prove otherwise. One answer to the puzzle, proposed by the Vienna Circle’s founder Moritz Schlick, is that math is true by definition: it follows from certain axioms by rules of deductive proof. But another member of the group, Kurt Gödel, showed that there are some seemingly true sentences about mathematics that cannot be proved using the usual axioms, and that adding extra axioms won’t help.

And if math is true by definition, whose definitions are we talking about anyway? Could I just redefine 2+2 to equal 5? Philosophers like Rudolf Carnap suggested that truth is relative to a set of definitions, and that we should choose our definitions on practical grounds. Our usual definitions of numbers ensure that 2+2 will always equal 4, and we should stick to those definitions because they’re useful in a broad range of fields, from accounting to engineering.

The philosophers of the Vienna Circle were constantly debating and refining the finer points of their ideas about meaning, observation, logic, and science. While the details of their views are contentious, I think they had some obvious insights that have changed philosophy for the better. They pointed to an important difference between theories in disciplines like chemistry, which can be tested by experiments, and horoscopes in the newspaper, which are vague enough to fit with nearly any observation—even if it’s a struggle to spell out exactly what that difference is. And they encouraged philosophers to give precise definitions for their terms—even if we sometimes disagree about when a definition is precise enough.

We’ll be talking more about the Vienna Circle this week with David Edmonds, author of The Murder of Professor Schlick: The Rise and Fall of the Vienna Circle and host of the Philosophy Bites podcast. I can’t wait to discuss the lives and ideas of this under-appreciated group of philosophers.