Friday, October 16, 2009

on poorly constructed thought experiments

For the background, we turn to the Stanford Encyclopedia of Philosophy's article on Nothingness:

.....

… scholars writing in the aftermath of the condemnation of 1277 proposed various recipes for creating vacuums. One scheme was to freeze a sphere filled with water. After the water contracted into ice, a vacuum would form at the top.

Aristotelians replied that the sphere would bend at its weakest point. When the vacuists stipulated that the sphere was perfect, the rejoinder was that this would simply prevent the water from turning into ice.

Neither side appears to have tried out the recipe. If either had, then they would have discovered that freezing water expands rather than contracts. […]

Hero was eventually refuted by experiments with barometers conducted by Evangelista Torricelli and Blaise Pascal. Their barometer consisted of a tube partially submerged, upside down in a bowl of mercury. What keeps the mercury suspended in the tube? Is there an unnatural vacuum that causes the surrounding glass to pull the liquid up? Or is there no vacuum at all but rather some rarefied and invisible matter in the “empty space”? Pascal answered that there really was nothing holding up the mercury. The mercury rises and falls due to variations in the weight of the atmosphere. The mercury is being pushed up the tube, not pulled up by anything.

When Pascal offered this explanation to the plenist Descartes, Descartes wrote Christian Huygens that Pascal had too much vacuum in his head. Descartes identified bodies with extension and so had no room for vacuums.

Descartes was not the only 17th-century wit to make this joke. Thus Thomas Pecke, "Upon Marcus", 1659:

Why durst you offer Marcus to aver

Nature abhorr'd a vacuum ? confer

But with your empty skull, then you'll agree

Nature will suffer a vacuitie.