One of the doctrines which was the subject of the warmest discussion between the defenders and opposers of Aristotle, at the revival of physical knowledge, was that in which he asserts,[28\1] "That body is heavier than another which in an equal bulk moves downward quicker." The opinion maintained by the **Aristotelians at the time of Galileo was, that bodies fall quicker exactly in proportion to their weight. The master himself asserts this in express terms, and reasons upon it.[29\1] Yet in another passage he appears to distinguish between weight and actual motion downwards.[30\1] "In physics, we call bodies heavy and light from their _power_ of motion; but these names are not applied to their actual operations (ἐνέργειαις) except any one thinks {72} _momentum_ (ῥοπὴ) to be a word of both applications. But heavy and light are, as it were, the _embers_ or _sparks_ of motion, and therefore proper to be treated of here." [Note 28\1: De Cœlo, iv. 1, p. 308.] [Note 29\1: Ib. iii. 2.] [Note 30\1: Ib. iv. 1, p. 307.] The distinction just alluded to, between Power or Faculty of Action, and actual Operation or Energy, is one very frequently referred to by Aristotle; and though not by any means useless, may easily be so used as to lead to mere verbal refinements instead of substantial knowledge. The Aristotelian distinction of Causes has not any very immediate bearing upon the parts of physics of which we have here mainly spoken; but it was so extensively accepted, and so long retained, that it may be proper to notice it.[31\1] "One kind of Cause is the matter of which any thing is made, as bronze of a statue, and silver of a vial; another is the form and pattern, as the Cause of an octave is the ratio of two to one; again, there is the Cause which is the origin of the production, as the father of the child; and again, there is the End, or that for the sake of which any thing is done, as health is the cause of walking." These four kinds of Cause, the _material_, the _formal_, the _efficient_, and the _final_, were long leading points in all speculative inquiries; and our familiar forms of speech still retain traces of the influence of this division. [Note 31\1: Phys. ii. 3.] It is my object here to present to the reader in an intelligible shape, the principles and mode of reasoning of the Aristotelian philosophy, not its results. If this were not the case, it would be easy to excite a smile by insulating some of the passages which are most remote from modern notions. I will only mention, as specimens, two such passages, both very remarkable.