Context: Mach suggested that thought experiments like the bucket argument are problematic. If we were to imagine a universe that only contains a bucket, on Newton's account, this bucket could be set to spin relative to absolute space, and the water it contained would form the characteristic concave surface. But in the absence of anything else in the universe, it would be difficult to confirm that the bucket was indeed spinning. It seems equally possible that the surface of the water in the bucket would remain flat.

Question: How did Mach describe thought experiments like the bucket argument?
Answer: problematic

Question: What is difficult to confirm about the bucket in the absence of anything else in the universe?
Answer: that the bucket was indeed spinning

Question: What was equally possible about the surface of the water in the bucket?
Answer: would remain flat.

Question: Who expanded on thought experiments?
Answer: Unanswerable

Question: What can be confirmed about the bucket independent of other objects in the universe?
Answer: Unanswerable

Question: What could not remain flat in absolute space?
Answer: Unanswerable