264 THE SCIENCE OF LOGIC 



inference is always practically unconscious, but it may be made ex 

 plicit in a syllogism of the second figure. Thus, if in the shadows 

 of the evening I see an object indistinctly looming into the field 

 of vision, I may think or judge immediately : &quot; That is a horse &quot;. 

 But the mental process is really this : 



&quot; A horse presents such and such appearances ; 



&quot; This object presents those same appearances ; 



&quot; Therefore this object is a horse.&quot; 



And it is manifest that this process will produce certitude only 

 where I can be sure that &quot; Whatever presents those appearances 

 is a horse &quot; ; in other words, where I can convert the major pre 

 miss simply, and so replace the formally invalid syllogism of the 

 second figure by a valid &quot; proof of fact &quot; in the first figure. If I 

 cannot do this, the &quot; inference of perception,&quot; as it is sometimes 

 called, will only produce probability. It is clearly of the same 

 form as the argument from analogy, and falls under the class of 

 argument known as the Aristotelean enthymeme, with which we 

 shall deal presently. 



The second source of probability mentioned above is human 

 testimony. When we cannot secure sufficient evidence, as to the 

 knowledge and trustworthiness of our authority, to produce moral 

 certitude, we must rest content with a probable assent to what 

 ever we accept on such authority : even though the natural yearn 

 ing of the mind is for a certain knowledge of the truth. Certitude 

 is not always attainable ; people often have to be content with 

 that high degree of probability which amounts to practical certi 

 tude. Hence it has been said that &quot; Probability is the guide of 

 life &quot; : The farmer sows his fields, the manufacturer erects his 

 machinery, the merchant opens his business, the soldier goes to 

 battle, young people marry, and statesmen legislate. Under the 

 influence of what determining motives ? What has the future to 

 offer them ? Hopes of success ; Probabilities}- 



As a fourth source of probability we have set down deductive 

 inference from premisses which are not all certain. If any one 

 premiss of an inference be only probable, the conclusion cannot 

 be more than probable : it must follow the weaker premiss. A 

 probable argument may be defined as an argument whose premisses 



1 &quot; Tola praesens vita per probabilitatem maxime ducitur. Relationes omnes 

 hominum in familia et in republica viventium, probabilitate fundantur. Qui scribit, 

 qui navigat, qui militat, qui uxorem ducit, et qui leges condit, nonnisi intuitu proba- 

 bilis eventus operatur.&quot; LEPIDI, Elementa Philosophiae Christianae, i., p. 318. 



