FALLACIES OF SIMPLE INSPECTION. 529 



left to himself is to amble, because otherwise he must either trot, gallop, or 

 stand still, and because we know no reason why he should do one of these 

 rather than another ? If this is to be called an unfair use of the " sufficient 

 reason," and the other a fair one, there must be a tacit assumption that a 

 state of rest is more natural to a horse than a state of ambling. If this 

 means that it is the state which the animal will assume when left to him- 

 self, that is the very point to be proved ; and if it does not mean this, it 

 can only mean that a state of rest is the simplest state, and therefore the 

 most likely to prevail in nature, which is one of the fallacies or natural 

 prejudices we have already examined. 



So again of the First Law of Motion ; that a body once moving will, if 

 left to itself, continue to move uniformly in a straight line. An attempt 

 is made to prove this law by saying, that if not, the body must deviate 

 either to the right or to the left, and that there is no reason why it should 

 do one more than the other. But who could know, antecedently to experi- 

 ence, whether there was a reason or not? Might it not be the nature of 

 bodies, or of some particular bodies, to deviate toward the right? or if the 

 supposition is preferred, toward the east, or south ? It was long thought 

 that bodies, terrestrial ones at least, had a natural tendency to deflect down- 

 ward ; and there is no shadow of any thing objectionable in the supposition, 

 except that it is not true. The pretended proof of the law of motion is even 

 more manifestly untenable than that of the law of inertia, for it is flagrantly 

 inconsistent ; it assumes that the continuance of motion in the direction first 

 taken is more natural than deviation either to the right or to the left, but 

 denies that one of these can possibly be more natui-al than the other. All 

 these fancies of the possibility of knowing what is natural or not natural 

 by any other means than experience, are, in truth, entirely futile. The real 

 and only proof of the laws of motion, or of any other law of the universe, is 

 experience ; it is simply that no otlier suppositions explain or are consistent 

 with the facts of universal nature. 



Geometers have, in all ages, been open to the imputation of endeavoring 

 to prove the most general facts of the outward world by sophistical reason- 

 ing, in order to avoid appeals to the senses. Archimedes, says Professor 

 Playfair,* established some of the elementary propositions of statics by a 

 process in which he "borrows no principle from experiment, but establish- 

 es his conclusion entirely by reasoning a priori. He assumes, indeed, that 

 equal bodies, at the ends of the equal arms of a lever, will balance one an- 

 other; and also that a cylinder or parallelopiped of homogeneous matter, 

 will be balanced about its centre of magnitude. These, however, are not in- 

 fei'ences from experience ; they are, properly speaking, conclusitos deduced 

 from the principle of the Sufficient Reason." And to this day there are 

 few geometers who would not think it far more scientific to establish these 

 or any other premises in this way, than to rest their evidence on that fa- 

 miliar experience which in the case in question might have been so safely 

 appealed to. 



§ 6. Another natural prejudice, of most extensive prevalence, and which 

 had a great share in producing the errors fallen into by the ancients in 

 .their physical inquiries, was this : That the differences in nature must cor- 

 respond to our received distinctions : that effects which we are accustom- 

 ed, in popular language, to call by different names, and arrange in different 



* Dissertation, p. 27. 

 34 



