hindered [by it]; for since the medium diminishes the heaviness of the mobile, it hinders its motion: upward motion in fact never takes place, unless it is helped by the medium. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.28.00.00"/>
<s id="id.5.0.28.00.01">To a positive effect there must exist a positive cause; hence the cause of motion cannot be lightness, which is a privation. {1}</s>
<s id="id.5.0.28.00.02">Hence it remains that it be heaviness: and that things that are moved upward are moved by heaviness. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.29.00.00"/>
<s id="id.5.0.29.00.01">We call local motion that [motion] in which the center of heaviness of the mobile is moved; hence we will not speak of the local motions of the celestial orbs since their center of heaviness, which is also their center of magnitude, always remains immobile. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.30.00.00"/>
<s id="id.5.0.30.00.01">If the corpulence and the density of water were the cause why wood is not submerged, undoubtably by the same cause, after it was submerged by something else, it would be hindered from coming back upward. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.31.00.00"/>
<s id="id.5.0.31.00.01">Aristotle has shown the falsity of Plato's excessive devotion to geometry, in Book I of De generatione et corruptione [ch. 2, 315b25-316a17]. {1}</s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.32.00.00"/>
<s id="id.5.0.32.00.01">Aristotle, in Book III, section 8 of the <i>Metaphysics</i> [ch. 2, 997a35-998a4], writes thus: For the lines perceptible to our senses are not such as geometry presupposes: for nothing that is perceptible to our senses is straight and curved in such a way: for the circle does not touch the rule at a point, but [it touches it] just as Protagoras said, in showing the falsity of the geometers. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.33.00.00"/>
<s id="id.5.0.33.00.01">Aristotle, in Book VII, text #10 of <i>Physics</i> [243a12-15], says that for the naturalness of motion an internal cause of motion, and not an external one, is required. </s>
</p>
</subchap2>
<subchap2>
<p>
<s id="id.5.0.34.00.00"/>
<s id="id.5.0.34.00.01">Arstotle, in Book III, text #72 of <i>De Caelo</i> [307a13-22], says: If it is because of the triangles that fire warms, it would follow that mathematical bodies would warm. </s>
