Lecture Notes by Christopher Lay

Los Angeles Pierce College

Department of History, Philosophy, and Sociology

Mill's System of Logic Book III: Of Induction, Chapter VIII:  "Of The Four Methods Of Experimental Inquiry"

( http://www.gutenberg.org/files/27942/27942-h/27942-h.html )

§ 1

Method of Agreement

"If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the instances agree, is the cause (or effect) of the given phenomenon." 

"[B]y comparing together different instances in which the phenomenon occurs," we have evidence pertaining to an effect's cause. 

Where ever we have an effect present in all instances, we can see how they further agree, and thus have evidence that the cause may be identical to that further agreement. 

Prior Factors	Effect
Cabbage, Pickle, Parsnip	Sick
Carrot, Pickle, Turnip	Sick
Onion, Pickle, Garlic	Sick
Tomato, Pickle, Beet	Sick
So _________ is the cause

§ 2

Method of Difference

"If an instance in which the phenomenon under investigation occurs, and an instance in which it does not occur, have every circumstance in common save one, that one occurring only in the former; the circumstance in which alone the two instances differ, is the effect, or the cause, or an indispensable part of the cause, of the phenomenon."

"[B]y comparing instances in which the phenomenon does occur, with instances in other respects similar in which it does not," we have evidence pertaining to an effect's cause. 

Where ever we have an effect present in merely one instance, we can see how that instance is further different, and thus have evidence that the cause may be identical to that further difference. 

Prior Factors	Effect
Skittles, Haggis, Jelly Beans	Not Sick
Skittles, Haggis, Jelly Beans	Not Sick
Skittles, McNuggets, Jelly Beans	Sick
Skittles, Haggis, Jelly Beans	Not Sick
So _________ is the cause

§ 4

Agreement and Difference

"If two or more instances in which the phenomenon occurs have only one circumstance in common, while two or more instances in which it does not occur have nothing in common save the absence of that circumstance, the circumstance in which alone the two sets of instances differ, is the effect, or the cause, or an indispensable part of the cause, of the phenomenon." 

Where ever we have an effect present in more than one instance, and less than all of the instances, we can see how those instances are in some further agreement and some further difference, and thus have evidence that the cause may be identical to that further agreement and difference. 

Prior Factors	Effects
Durian, Blueberry, Apple	Sick
Durian, Raspberry, Apple	Sick
Durian, Orange, Pineapple	Not Sick
Durian, Peach, Pineapple	Not Sick
So _________ is the cause

§ 6

Concomitant Variation

"Whatever phenomenon varies in any manner whenever another phenomenon varies in some particular manner, is either a cause or an effect of that phenomenon, or is connected with it through some fact of causation." 

Consider the tides and the phases of the moon.