GALILEO AND THE ARISTOTELIAN PHYSICS

The scope of what Koyré termed ´the Scientific Revolution of the 17^th century´ in the Galileo Studies was confined in its usage, solely, to the activities of Galileo and Descartes. Galileo was the first scientist to realize the idea of a mathematized physics while Descartes was the first to think up, in a systematic way, a world consisting of nothing but bodies moving through infinite space in accordance with the principle of inertia. However, in the Newtonian Studies he incorporated a third actor, Isaac Newton, who completed the revolution circle by unifying the corpuscular philosophy of Gassendi, Roberval, Boyle and Hooke with the panmathematism of Galileo and Descartes.¹⁰⁶

Koyré emphasis on the ´mutation´ the human mind underwent on its way to achieving the modern science of mechanics was to capture the immensity of the ingenuity employed to overcome the intellectual hurdles posited by the Aristotelian natural motion. The Aristotelian natural theory asserted that all heavy terrestrial bodies had a natural motion towards the centre of the universe. This implies that motion in any other direction was violent motion because it contradicted the ordinary tendency of a body to move to what was regarded as its natural place. Such motion depended on the operation of a mover. Therefore, a body would keep in movement only so long as a mover was actually in contact with it, imparting motion to it steadily. Once the mover stops to prompt the movement of the body it falls straight to the centre of the earth which is its resting place.

105Alexander Koyré From the Closed World to the Infinite Universe, p. v

106Alexandre Koyré, Newtonian Studies (Chicago: University of Chicago Press, 1965) p.12

In its own right, the Aristotelian theory does not sound stupid because it corresponds in a self-evident manner with most of the data available to common sense. However, there are facts which cannot square well with it even when analysed on the common-sense level. For instance, on the common-sense level the Aristotelian theory imply that an arrow ought to have fallen to the ground the moment it lost contact with the bow-string.

Naturally, it should be so since neither the bow-string nor anything else could impart a motion which would continue after the direct contact with the original mover had been broken. Therefore, this theory cannot make a logical claim on how an arrow fired into the wind could be carried forward by the reaction of the air.

The Aristotelian theory of natural motion held sway for centuries before the advent of the Scientific Revolution not because its inconsistencies were not noticed but because there was no immediate alternative since the colossal intellectual system to which it was a part gained hold on medieval scholastic thought. Nevertheless, the Aristotelian teaching carried such an intricate dovetailing of observations and explanations which was hard for the human mind to escape from. Reconstructing the deficiencies of the Aristotelian motion meant dismantling the whole natural philosophy on which it was built. The cosmos of the Aristotelian natural philosophy is ´a finite and hierarchically ordered, therefore qualitatively and ontologically differentiated universe.¹⁰⁷

The Parisian terminists who were the first men in the middle ages to launch great attack on the Aristotelian theory were also conscious of the fact that this colossal issue was involved in the task. Such men like Jean Buridan and Nicholas of Oresme pointed to alternative interpretation that would eliminate the need for the Intelligences that turned the universe. This alternative was contained in the impetus physics of the Parisian tradition. The impetus theory conceives motion as the effect of a force contained within the moving body. Koyré highlighted the importance of this theory in creating the first departure from Aristetolianism. He stated that impetus physics makes it possible to separate the body´s motion from the idea of the goal, towards which it is directed, and makes it possible to isolate the moving body from the rest of the Universe.¹⁰⁸

107Ibid., p. 7

108Alexander Koyré, Galileo Studies, p. 70

The theory of impetus takes motion to be the effect produced by a cause internal to the moving body. Impetus could then be identified as an efficient cause producing motion as its effect. With the impetus theory the Parisian terminists especially Giordano Bruno sought to replace Aristotle´s close and finite world with an open and infinite universe and this involved the rejection of the notion of ´natural´ and ´violent´ motions. In some ways, this impetus-dynamics helped the Parisian terminists to displace the Aristotelian dynamics. The theory was able to refute some of the arguments of Aristotle. ´Yet it was not able to meet all of them; still less was it able to carry the structure of modern science.´¹⁰⁹

Similarly, the Galilean physics rejected the notion of the ´natural´ and ´violent´ motions.

This physics maintained that a motion never reveals nor expresses the nature of a body.

Motion does not affect the moving body in itself and is only possessed by the moving body in relation to something other than itself. For Galileo, gravity or ´heaviness´ is the only natural property of bodies and is also the only natural source of motion. This implies that since gravity or heaviness is a universal natural property of all bodies, it is what produces in all bodies a natural motion in a downwards direction. As such fall is a natural and universal motion. However, Galileo does not take gravity as a natural quality of bodies. Though gravity refers to something in reality which he was not capable of mentioning, he restated that it does not constitute the ´nature´ of bodies neither is it one of their essential properties. It is rather an empirical property than a theoretical property of bodies. Its empiricity is derived from common sense. As such it is just a name assigned to ´downwards tendency´ of natural bodies. Galileo identified the essence of bodies as their mathematical properties. Koyré summaries thus,

It can be seen that in Galileo´s view (as in Descartes´, and for the same reasons) what constitutes the essence of bodies, or of matter, that which we cannot think of them as being without, and consequently that without which they could not exist, are their mathematical properties.¹¹⁰

109 Alexandre Koyré, ¨Galileo and the Scientific Revolution of the Seventeenth Century¨, in The Philosophical Review (Ithaca: Duke University Press, 1943) p.342

110Alexander Koyré, Galileo Studies, p. 179

In the actual reality the mathematical idea Galileo propounds does not designate any essential quality of bodies. It should be noted that he was not discussing real bodies as we definitely observe them in the real world. The bodies to which he makes such mathematical attributes are geometrical bodies moving in a world without resistance and without gravity. The type of world implied here is that boundless emptiness of Euclidean space which Aristotle had regarded as unthinkable by his strong rejection of the existence of a vacuum. Herbert Butterfield reiterated that even the ´Aristotelians regarded ´such´ a complete void as impossible, and said that God Himself could not make one´.¹¹¹ For Koyré, this ingenious effort of Galileo in establishing a different wider framework to account for the local motion needed a ´veritable mutation in human thought´. Therefore, The Scientific revolution of the seventeenth century was without doubt such a mutation.¹¹² The ´mutation´ that Koyré had in mind here was a product of the geometrization of space and the dissolution of the cosmos, both contained in the principle of inertia, first stated by Galileo and fully articulated by Descartes.