Over Descartian vortices you hover. And perhaps, at mid-day, in the fairest weather, with one half-throttled shriek you drop through that transparent air into the summer sea, no more to rise for ever.
You’ve got your head in the clouds, thinking big thoughts about 17th Century mathematician and philosopher Rene Descartes' principles of motion, which say that all motion can be charted on graph paper (basically), until all of a sudden you lose your footing and plummet through the air (which doesn’t look at all like graph paper) from the mast-head to a watery grave.
Descartes is the perfect reference here because his BIG idea, Cartesian Dualism, holds that the mental and material are of a different substance; that the mental does not occupy space, and that the material cannot think, which resonates with the tension between the practical/physical and the philosophical/abstract that Melville has been playing with throughout the chapter. And also because one of his most famous thought experiments, the Cartesian diver, demonstrates the principle of buoyancy. Get it?
Rene Descarte, looking french
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