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Thomas Hobbes adheres to a conception of philosophy as causal knowledge that bears the mark of the Aristotelian tradition, as Cees Leijenhorst has elaborated in another issue of The Monist. Referring to Aristotle, Hobbes states explicitly in two mathematical studies of the 1660’s: “To know is to know by causes.” But according to Hobbes, we encounter obstacles when we search for causes in the field of natural philosophy. Consequently, his well-known definition of philosophy consists of two parts. The earliest version, (...) elaborated in the so-called A 10 manuscript, reads: “Philosophy is the knowledge, acquired by correct ratiocination, of properties of bodies from their conceived manners of generation, and again, of such manners of generation, as may be, from known properties.” This definition reappears almost literally in several of Hobbes’s other essays, for example in De Cive and in the Leviathan, and in a modified version in De Corpore. In the latter, he replaces “properties of bodies” by “effects or appearances” and “manners of generation” by “causes or manners of generation.” But one essential point remains unchanged: the second part of the definition is not a simple reversal of the first part, since Hobbes does not speak of “manners of generation” or of “causes,” but only of possible manners of generation or of possible causes. It is this second part of Hobbes’s theory that refers to natural philosophy. Hobbes stresses repeatedly that in the case of natural philosophy, causes can be concluded only from known effects. In contrast, in First Philosophy or geometry, conclusions can be drawn from known and true principles to their effects. According to Hobbes, this is the essential distinction of natural philosophy, which does not exist by accident. This essay explicates Hobbes’s argumentation for claiming this distinction for natural philosophy but not for geometry. In addition, the consequences of Hobbes’s methodological argument for natural philosophy are examined, especially the consequences for optics and astronomy. Finally, the question of whether there is a development in this theory will be explored. (shrink)

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In the field of astronomy, Thomas Hobbes's mechanistic philosophy was influenced by Johannes Kepler. Whereas Galilei still sticks to the circular motion of the planets, Hobbes takes over the Keplerian ellipses. According to Kepler, he defines astronomy as ' celestial physics'. As a consequence, he tries to determine the cause for the planetary motion and the reason why the orbit of the earth is eccentric. Hobbes modifies Kepler's explanation given in the Epitome astronomiae Copernicanae that the earth consists of two (...) parts: one well-disposed, the other hostile towards the sun. Referring to this doctrine, Hobbes developed various astronomical theories throughout a time span of about 35 years in works like "De motu" , De corpore , and the "Decameron physiologicum". (shrink)

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At the beginning of the seventeenth century, the sine law of refraction had been discovered. Thus, natural philosophers tried even more to find a cause of refraction and to demonstrate the law. One of them was Thomas Hobbes, who was the author of the Leviathan and also worked on optics. At first, in the Hobbes analogy , he was influenced by Ibn al-Haytham, just as Descartes was in his famous proof in the Dioptrique . In his later optical scripts Tractatus (...) Opticus I , Tractatus Opticus II , and A Minute or First Draught of the Optiques , he developed a new explanation. Rejecting a corpuscular theory of light, Hobbes conceived a ray not as a body but as a motion originating from the light source: a ray can only be the motion of a body. The normal to the sides of a ray is called 'linea lucis'. If a ray is incident into another medium with a different density, one part of the linea lucis will be in the rarer and the other in the denser medium during an imperceptibly short period. Because the resistances in the two media are different, the parts of the linea lucis will move with different velocities; as a result the linea lucis will rotate and the direction of the ray will be changed. The next explanation given in De Corpore comes closer to the first one that Hobbes set down in the analogy. It must be asked why he replaced the theory of rotation by one which seems to carry less conviction. The reason could be that the dropped theory is founded in part on basic requirements of a corpuscular theory of light. Abandoning the whole theory might have been the lesser evil for Hobbes. In two later works, the Problemata Physica and the Decameron Physiologicum , Hobbes varied his explanation without giving any proof for the sine law. It should be noted, however, that in the Decameron he refers to the proof contained in the Tractatus Opticus I, but not that given in De Corpore. (shrink)

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