3. METHOD OF DIFFERENCE.

(1) Principle stated.

Says Mill, “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 of an indispensable part of the cause, of the phenomenon.”

To put this in simple terms: Whatever is invariablypresent when the phenomenon occurs and invariably absent when the phenomenon does not occur, other circumstances remaining the same, is probably the cause or the effect of the phenomenon.

(2) Method symbolized.

Using the same symbols as were used in “Agreement.”

Antecedents Consequents

A B C D P

– B C D –

P A B C D

– – B C D

In the first instance A is probably the cause of the phenomenon, since it is present when the phenomenon occurs and absent when it does not occur. For a similar reason, A is the effect in the second case.

(3) Concrete illustrations.

(A) A wise teacher in ascertaining the cause of John’s tardiness would have suggested at once a change of route. Using as symbols the initial letters of the key-words of the antecedents in the case, the following results:

s d h w t

s d h − –

(B) First trial.

Problem: Unprepared home work.

Antecedents		Consequents
1. Length of lesson,		Work not properly prepared.
2. Definiteness of lesson,
3. Amount of interest shown,
4. Physical condition the same.

Second trial.

1. Length of lesson the same,		Work properly prepared.
2. Lesson made more definite,
3. Interest the same,
4. Physical condition the same.

The foregoing symbolized:

L D I C W

L – I C —

It is seen that indefiniteness of lesson assignment is the cause of the unprepared home work.

(4) Advantages and disadvantages of the Method of Difference.

The main difficulty attending the use of the method of difference is the complexity of phenomenon. The very nature of the method insists as an essential requirement that only one material antecedent shall be varied at a time. In life the variations are more or less confused, and it is often not only impossible to observe cases of a single variation, but frequently error comes through overlooking antecedents which are material to the case under investigation. For these reasons the Method of Difference is more a method of experiment than it is a method of observation. By controlling the circumstances it becomes possible to vary but one antecedent at a time, and also to bring into prominence all of the material antecedents.

Bacon claims that all “crucial instances” are merely applications of the Method of Difference. By crucial instance he means any fact which will enable us to determine at once which supposition is the correct one. Forexample, the physician may not know whether it is malaria or typhoid fever till he takes a blood test; such a test typifies “crucial instances.” The various tests in chemistry are likewise cases of crucial instances, and, in consequence, this science makes use of “Difference” more than any other method.

(5) Characteristic features of Method of Difference.

There are three distinguishing marks of the Method of Difference: these are, (1) The phenomenon does not always happen; (2) One antecedent is variable; (3) The other antecedents are more or less invariable.

The following symbolizations will make these three characteristics evident:

	Antecedents	Consequents
(1)	A B C	P
	A – C	–
(2)	– B C	–
	X B C	P
(3)	L M T K	P
	L M – K	–

AGREEMENT AND DIFFERENCE COMPARED.

(a) The methods of Agreement and Difference are complementary as may be discerned by comparing their characteristic features: In Agreement the phenomenon always occurs; in Difference the phenomenon does not always occur: In Agreement there is one invariable antecedent; whereas in Difference there is one variable antecedent: In Agreement the other antecedents are more or less variable; but in Difference the other antecedents are more or less invariable.

(b) According to Mill the Method of Agreement insists that what can be eliminated is not connected; whereas the Method of Difference implies that what cannot be eliminated is connected.

(c) The Method of Agreement is more a method of observation, since it is chiefly concerned with the discovery of causes. The Method of Difference is distinctly a method of experiment, because its usual aim is to discover effects.

(d) The Method of Agreement is so called because the object is to compare several instances to determine in what respect they agree; but in the case of Difference instances are compared to determine in what respects they differ.

(e) The conclusions of the Method of Difference involve greater certainty than those of Agreement and, therefore, the former method should be adopted when there is a choice.