Sometime in the late 1650s, Robert Boyle built an apparatus that removed the air from within a glass dome. The members of the newly-formed Royal Society promptly set about devising all manner of experiments to perform with the newfangled device. They placed candles, mercury barometers, and then – just as one might expect of unsupervised boys – living mice within the dome and watched what happened as a piston systematically drained the air away. (It is probably a good thing for the local fauna that the experimenters did not have a bigger air pump at their disposal.)
The effects of these experiments were easily seen, but controversies raged nonetheless over what was really going on. Did the device truly empty everything from the dome? Thomas Hobbes, Henry More, and others insisted that a pure vacuum is impossible, and that some sort of rarified matter must remain in the dome. Their reasons for asserting this so confidently are various. Some found the suggestion of a specific volume attached to no material thing unintelligible. Others found it ridiculous to believe that as Boyle expelled the air out of the dome, and nothing took its place, the volume of the universe would necessarily inflate by just that amount.