 Indeed, where there are properly philosophical arguments to be had for a thesis, experimentation is superfluous. In assessing Boyle's experiments designed to prove that all tangible properties depend on the mechanical features of objects, Spinoza wonders why Boyle bothered, since this conclusion “has already been abundantly proved by Verulam, and later Descartes”. (Letter 6). Descartes had proved this a priori from the fact that the sole nature of body is extension, whose sole attributes are size, shape and motion. Moreover, ordinary, mundane observations offer as good proof as any that might be afforded by controlled observation. Boyle's careful experiments could not add any weight to the evidence already available from such ordinary phenomena as the facts that even cold sticks will spark a fire when rubbed together, that water makes sound when it comes to a moving boil, and that stirring and warming foul smelling bodies make them smell yet worse. Spinoza's attitude towards experimental observation seems to be, then, that it can have no weight against a theory based in sound a priori philosophical intuition and geometrical demonstration, and is easy available, though not necessary, to confirm the results of such pure theorizing. Experimentation can help us discover new phenomena, but it cannot help us to prove any scientific propositions we do not already know to be true. As Gabbey (1996) puts it, experimentation “cannot uncover the nature of things; sensory knowledge belongs to the imagination, the knowledge of essences and causes to the intellect alone” (Gabbey 1996, p. 171).