44 DISSERTATION SECOND. [part i. 



This, too, is a work not peculiar to the philosopher, but, in 

 a certain degree, is performed by every man who compares 

 one thing with another, and who employs the terms of or- 

 dinary language. 



Another great branch of knowledge is occupied, not 

 about the mere arrangement and classification of objects, 

 but about events or changes, the laws which those changes 

 observe, and the causes by which they are produced. In 

 a science, which treated of events and of change, the nature 

 and properties of motion came of course to be studied, and 

 the ancient philosophers naturally enough began their in- 

 quiries with the definition of motion, or the determination 

 of that in which it consists. Aristotle's definition is highly 

 eharacteristical of the vagueness and obscurity of his phy- 

 sical speculations. He calls motion "the act of a being in 

 power, as far as in power," — words to which it is impossi- 

 ble that any distinct idea can ever have been annexed. 



The truth is, however, that the best definition of motion 

 can be of very little service in physicks. Epicurus defined 

 it to be the " change of place," which is, no doubt, the 

 simplest and best definition that can be given ; but it must, 

 at the same time, be confessed, that neither he nor the 

 moderns who have retained his definition, have derived the 

 least advantage from it in their subsequent researches. 

 The properties, or, as they are called, the laws of motion, 

 cannot be derived from mere definition ; they must be 

 sought for in experience and observation, and are not to be 

 found without a diligent comparison, and scrupulous exami- 

 nation of facts. Of such an examination, neither Aristotle, 

 nor any other of the ancients, ever conceived the necessity, 

 and hence those laws remained quite unknown throughout 

 all antiquity. 



When the laws of motion were unknown, the other parts 

 of natural philosophy could make no great advances. In- 



