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Publication Information: Article Title: Hobbes on Hypotheses in Natural Philosophy. Contributors: Frank Horstmann - author. Journal Title: The Monist. Volume: 84. Issue: 4. Publication Year: 2001. Page Number: 487+. COPYRIGHT 2001 Hegeler Institute; COPYRIGHT 2004 Gale Group Hobbes on Hypotheses in Natural Philosophy. by Frank Horstmann INTRODUCTION Thomas Hobbes adheres to a conception of philosophy as causal knowledge that bears the mark of the Aristotelian tradition, as Cees L
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  Publication Information: Article Title: Hobbes on Hypotheses in Natural Philosophy. Contributors: Frank Horstmann - author. Journal Title: The Monist. Volume: 84. Issue: 4. Publication Year: 2001. Page Number: 487+. COPYRIGHT 2001 Hegeler Institute; COPYRIGHT 2004 Gale Group  Hobbes on Hypotheses in Natural Philosophy. by Frank Horstmann INTRODUCTION 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. (1) Referring to Aristotle, Hobbes states explicitly in two mathematical studies of the 1660's: To know is to know by causes. (2) 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 (ca. 1639), 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. (3) This definition reappears almost literally in several of Hobbes's other essays, for example in De Cive and in the Leviathan, (4) 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. (5) 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. (6) Finally, the question of whether there is a development in this theory will be explored. II HOBBES'S ARGUMENTS FOR HIS METHODOLOGICAL DISTINCTION BETWEEN NATURAL PHILOSOPHY AND OTHER SCIENCES The initial document in which Hobbes elucidates his theory of natural philosophy is a letter written in August 1636 to William Cavendish, Earl of Newcastle: In thinges that are not demonstrable, of [w.sup.ch] kind is [y.sup.e] greatest part of Naturall Philosophy, as dependinge vpon the motion of bodies so subtile as they are invisible, such as are ayre and spirits, the most that can be atteyned vnto is to haue such opinions, as no certayne experience can confute, and from [w.sup.ch] can be deduced by lawfull argumentation, no absurdity (...) (7) This brief passage contains four essential points that combine to form a theory of method in nuce. Hobbes first claims that the greatest part of natural philosophy is not demonstrable. Second, he bases his assertion on the motion of invisible bodies and adds the variety of thinges is but variety on the motion of invisible bodies and adds the variety of thinges is but variety of locall motion in [y.sup.e] spirits or invisible partes of bodies. (8) Third he claims that only a limited degree of certainty can be obtained in natural philosophy, and in the last claim he specifies the conditions for arriving at this degree. Relating to the first, third and fourth points, only marginal modifications are to be found in Hobbes's later works. With regard to the second point, however, Hobbes makes a noteworthy change. In the Tractatus Opticus II, written circa 1640, Hobbes states that the methodological treatment of natural things is different from the methodology of other sciences. In the other sciences principles are employed that are made true by us. In order to provide explanations of natural causes it is necessary to utilise another kind of principle: hypotheses. (9) In a letter to Charles Cavendish in February 1641, Hobbes writes that in the mathematical sciences we arrive at a  final definition by way of a principle, which is made true by pact and consent amongst our selves. (10) In the field of natural philosophy it is not possible to make true principles through definition, which does not however exclude another eventuality for arriving at true principles. For realising the essential and principal distinction between natural philosophy and other sciences we have to return to the above quoted passage from A 10. To achieve knowledge of properties of bodies from their conceived manners of generation, we first of all have to know these manners of generation. But if we conclude from known properties the manners of generation, it is true that we could get a possible manner of generation; however, we cannot be sure whether this is the only possible manner of generation: similar appearances could be produced by dissimilar motions. (11) Like Descartes in his clock analogy, (12) Hobbes confesses: For there is no effect in nature which the Author of nature cannot bring to pass by more ways than one. (13) In a famous statement given in the epistle of dedication to the Six Lessons from 1656, Hobbes explains that in his opinion we know the manners of generation in fields like geometry and in political philosophy, but not in natural philosophy: Of arts, some are demonstrable, others indemonstrable; and demonstrable are those the construction of the subject whereof is in the power of the artist himself, who, in his demonstration, does no more but deduce the consequences of his own operation. The reason whereof is this, that the science of every subject is derived from a procognition of the causes, generation, and construction of the same; and consequently where the causes are known, there is place for demonstration, but not where the causes are to seek for. Geometry therefore is demonstrable, for the lines and figures from which we reason are drawn and described by ourselves; and civil philosophy is demonstrable, because we make the commonwealth ourselves. But because of natural bodies we know not the construction, but seek it from the effects, there lies no demonstration of what the causes be we seek for, but only of what they may be. (14) Hobbes--in the tradition of maker's knowledge (15)--claims true principles both for geometry and for political philosophy because we describe geometrical figures, and because the commonwealth and the causes of justice are produced by us. (16) With regards to geometry, a circle in which the knowledge of an effect is obtained from the knowledge of the construction illustrates the point. The circle is produced by the circumduction of a body of which one end remains unmoved; the length of the body is not changed. When it is carded around it successively settles on all radii; because of the constant length all radii are equal, and furthermore, because of the unmoved end a figure is produced having one centre and a circumference of which all points are reached by equal radii. (17) It would appear that Hobbes attempts to explain the hypothetical character of natural philosophy by invisible motions as he does in the letter to William Cavendish from 1636--an interpretation advanced by many Hobbes scholars. (18) To take issue with one of these interpretations, I would argue that Douglas M. Jesseph is only partly correct when he claims that for Hobbes there is no science of `sensible appearances' because the specific causal mechanisms that produce such appearances remain hidden from our view. (19) A passage out of the Decameron Physiologicum seems to support Jesseph's opinion: For you enquire not so much, when you see a change of anything, what may be said to be the cause of it, as how the same is generated; which generation is the entire progress of nature form the efficient cause to the effect produced. Which is always a hard question and, for the most part, impossible for a man to answer to. The alterations of the things we perceive in our five senses are made by the motion of bodies, for the most part, either for distance, smallness, or transparence, invisible. (20) But Hobbes only permits that we could know the manners of generation, despite that we do not produce them, if we could perceive them. The matter becomes clearer by consideration of a passage out of De Homine where Hobbes says that, in contrast to geometry, the causes of natural things are not known to us because they (the causes) are not in our power and because most of them are invisible. (21) However, he does not say that we cannot know them because we cannot perceive them. In spite of the inacessibility of invisible motions to our senses, we can nevertheless not exclude the possibility that the true causes of sensible appearances could be derived by ratiocination. However, according to Hobbes this is not possible because we have not produced the natural effects, and, therefore, we have to conclude from effects to possible causes. In this context there is a noteworthy remark in the Examinatio et Emendatio. The interlocutor claims physics is not demonstrable because of the imperceptibility of natural actions. The objection given to this argument is that although it is also not possible to perceive the accuracy of the ratio between two lines or two magnitudes, ratio itself is nevertheless demonstrable. (22) Therefore, invisibility alone is not a sufficient reason for the assertion that physics is not  demonstrable. Moreover, the visibility of a motion does not imply that we know the motion exactly: within the framework of the above-mentioned construction of the circle, Hobbes says that in the case of a plane figure coming very close to a circle we cannot know with certainty based on our senses whether the figure is a circle or not. (23) Hobbes occasionally refers to the demonstrability of physics in the concluding section of De Corpore and in De Homine. (24) But this is admittedly correct only with the reservation that the causes that the so-called demonstrations are founded on are only possible causes which may be true. And we cannot know if they are true. (25) We may admit that Hobbes, when speaking of demonstrations, expresses himself sometimes in an ambiguous manner. What remains without doubt is that according to Hobbes only conclusions drawn from known causes to effects are genuine demonstrations. (26) It is true that he says in De Homine that both conclusions from the causes to the effects and from the effects to the causes are called demonstrations, but he adds that only the former deserve to be called demonstrations. (27) As a result, it must be concluded that for Hobbes there is a clear distinction between geometry, which can be demonstrated from known principles made by us, and physics, which depends on hypotheses. III THE HYPOTHETICAL CHARACTER OF NATURAL PHILOSOPHY AND ITS CONSEQUENCES Claiming that only hypothetical explanations can be given in the field of natural philosophy is not  just lip service for Hobbes. For example, his Tractatus Opticus I starts with five hypotheses from which 14 propositions are deduced. (28) In the chapter On the Universe and the Stars of De Corpore the explanations of astronomical phenomena are based on six hypotheses for describing the phenomena of nature. (29) And in his poem De Motibus Solis, Aetheris & Telluris Hobbes already confesses explicitly in the title De Motibus Solis, Aetheris & Telluris. Praecipue autem Numeri Dierum In hemisphaero Boreo, quam in Australi Majoris, Causa conjecturalis that he is only suggesting a possible cause for the eccentric path of the Earth. (30) However, in order to avoid misunderstandings it is important to point out that according to Hobbes, although we cannot know true causes in physics, this does not imply that all supposed causes are inherently equal in terms of their value for providing explanations. As stated above, Hobbes makes this clear in his letter of August 1636 to William Cavendish. Hobbes judges the hypotheses concerning light made by Walter Warner and Claude Mydorge to be uncertain and not true, in contrast to the opinion of William Cavendish--and of Hobbes himself--which read that the variety of thinges is but variety of locall motion in [y.sup.e] spirits or invisible partes of bodies. (31) But what criteria does Hobbes employ for claiming a supposition to be an acceptable supposition? Hobbes's criteria in effect may seem to be lacking when he writes the most that can be atteyned unto is to have such opinions, as to certayne experience can confute, and from [w.sup.ch] can be deduced by lawfull argumentation, no absurdity [...] (32) Cum granto salis this criteria has not been changed. Similarly, Hobbes states, in the Tractatus Opticus II, that we can only demand from a physicist that he assume imaginable motions, that the necessity of the appearances can be demonstrated by these concessions and therefore, that nothing can be derived which is wrong. (33) In De Corpore he claims that he has used only hypotheses which are plausible and can be easily comprehended and that he has concluded rightly from these assumptions. (34) Consistently, Hobbes writes in 1661 that the law of all hypotheses reads that all which is supposed has to be possible and cogitable, (35) and that a right hypothesis has to be conceivable that is not absurd, and that by conceding it the necessity of the appearance can be concluded. (36) It may be objected that Hobbes provides feeble criteria, and for example, the only negative criterion for the conceivability of a hypothesis, that it must not be absurd, is vague, unsatisfactory, highly subjective, and scarcely suitable to exclude in a convincing manner hypotheses assumed by others. It is sufficient to repudiate objections made by himself, but it seems not to be even sufficient to justify Hobbes's well known polemics against the empty words of the Scholastics, such as substantial forms, incorporeal substances, antiperistasis, antipathy, and sympathy. (37) In order to address this objection to Hobbes's criteria we have to take into consideration that the only hypotheses granted by Hobbes are those which comply with another qualifying condition. The hypotheses have to agree with the theorems of his First Philosophy and geometry, especially with one of the main principles of Hobbes's natural philosophy: all action is motion of a body caused by contact with another moved body. (38) Reducing hypotheses to this and other mechanical principles is in fact very restricting. However, Descartes, who assumes that there is an inclination to motion which is not a real motion, denies this principle--that all action is motion--which is accepted by Hobbes. (39) It is  also necessary to ask, On what basis is it possible to object to Kepler's concept of an immaterial species that only a moved and contacting body can move another body? (40) One may share Hobbes's opinion that such an inclination is incogitable and that Kepler's phrase immaterial species is nothing but an inconceivable connection of words. (41) Furthermore, one may be convinced by the justification of the principle given in De Corpore (42)--but in the end the whole building would collapse without that cornerstone. Therefore, the anchoring of Hobbes's hypotheses on the principles of his First Philosophy and geometry seems to be of little use, if these principles themselves are doubted. But it is a trivial, even if true, remark that unproven basic principles are used, and that this weakness of any system of thought is inevitably a weakness of Hobbes's physics. In this case it may carry more weight, because for a lot of reasons he seems to be hardly interested, as Frithiof Brandt claims, in verifying his hypotheses. (43) It cannot be denied that Hobbes's physics lacks the essential element of verification by a great number of thorough and accurate observations. Nevertheless, I would argue that Brandt is only partly right in his critique of Hobbes. We should not neglect to point out that Hobbes's First Philosophy and geometry precede his physics, and harmonising the hypotheses of the latter with the former--which for Hobbes are demonstrated and true hypotheses--may also be conceived as a verification of the hypotheses of his physics. According to Hobbes, observations and experiments are not missing in verifications by coincidence, because Hobbes disapproves of the emerging experimental science in England, a fact demonstrated by his well-known controversy with Robert Boyle. (44) But in spite of Hobbes's scoffing comment that, according to the new theory of experimental science, (45) pharmacists would be the best of all physicians, he does not disdain experiments. Rather, he points out the importance of the correct interpretation of experiments, which is not achieved by a greater number of experiments, but by a better understanding of them. Again, Hobbes agrees with Aristotle, namely, that to be ignorant of motion is to be ignorant of nature. Therefore, he reproaches the experimentally oriented scientists for ignoring motion. Despite their numerous experiments, they will make no scientific progress without accepting Hobbes's principles. (46) It may come as a surprise therefore that in this context Hobbes refers to the knowledge of motion as the noblest part of geometry, (47) and it is therefore necessary to explain in which way Hobbes reduces physical hypotheses to the principles of geometry. For this we have to look to the third part of De Corpore which is entitled On Proportions of Motions and Magnitudes and which is the connecting link of his First Philosophy and his physics. At first glance we may agree that in this section of the text both geometrical and mechanical chapters are mixed together. (48) because alternately we read chapters on motion and on geometrical figures. George C. Robertson, critical of such a mixture, judges it as an ill-ordered section [...] that we shall best, in attempting to pass as swiftly as possible over this portion [...]. (49) Indeed, this part of De Corpore is very strange to us, because of Hobbes's conception of geometry. As we have seen, a circle is generated by motion of a body, as is the case for all geometrical figures. This means that geometry is founded on the knowledge of motion. However, not only geometry is founded on the knowledge of motion, but also mechanics, which Hobbes elaborates in the third part of the text. In these chapters on mechanics Hobbes demonstrates, from the known manner of generation, the path of a moved body. Hobbes first defines simple circular motion as that motion in which several points of a moved body describe several equal arcs simultaneously. He then demonstrates that every straight line taken in a body moved with simple circular motion is always carried parallel to itself. (50) Furthermore, he demonstrates, that, if in a fluid medium a spherical body is moved with simple circular motion, then for another spherical body, given that it is not fluid, a simple circular motion will also move it. (51) Finally, he demonstrates that the path of the second body will not be a perfect circle, if the parts of this body do not have the same degree of liquidity. (52) So Hobbes has provided a possible source of generation of the annual motion of the Earth around the sun. Therefore, when approaching questions of astronomy he is able to explain the Earth's orbit on an elliptical path with a simple reference to the geometrical mechanics elaborated in this chapter. (53) He has merely to assume a simple circular motion of the sun and a fluid medium, namely the ether, in which both the sun and the Earth are located. Geometrical mechanics, therefore, supplies physics with possible forms of generation, which can be used for explanations, but only for hypothetical explanations. In the example of the Earth's orbit, we know that a simple circular motion of the sun would generate a simple circular motion of the Earth, which results in an elliptical path because of the different degrees of liquidity of the southern and the northern hemispheres of the Earth. We have to conclude however, from the
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