3. _A fact must follow a certain law, because we see no reason for its deviating from it in one way rather than in another._ This, which is the same as the Principle of the Sufficient Reason, has been used to prove the Law of Inertia (the very point to be proved, viz. that only external force can be a sufficient reason for motion _in a particular direction_, being assumed), and also the First Law of Motion, the argument being, in the latter case, that a moving body, if it do _not_ continue of itself to move uniformly in a straight line, must deviate right or left, and that there is _no reason_ for its going one way more than the other: to which the answer is, that, apart from experience, we could not know whether or not there were a reason. Geometers often fall into this Fallacy. 4. _The differences in nature must correspond to our received distinctions_ (in names and classifications). Thus, the Greeks thought that, by determining the meanings of words, they ascertained facts. Aristotle usually starts with 'We say thus or thus.' So, with the _Doctrine of Contrarieties_, in which the Pythagoreans and others assumed that oppositions in language imply similar ones in nature. Hence, too, the ancient belief in the essential difference between the laws of things terrestrial and things celestial, and in man's incapability of imitating nature's works. Bacon's error (which vitiates his inductive system) was analogous, in looking (either through his eagerness for practical results, or a lingering belief that causes were the sole object of philosophy) for the cause of given effects rather than the effects of a given cause. Hence sprang his tacit assumption (and that in enquiries into the causes of a thing's sensible qualities, where it was especially fatal), that in all cases, e.g. of heat or cold, the _forma_, or set of conditions, is _one_ thing. A similar notion, viz. that each property of gold, as of other things, has its one _forma_, produced the belief in Alchemy.