“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”.
The importance of this method is that the causal connection is determined on the basis of double agreements i.e., the agreement in presence and agreement in absence.
Hence for this method there is the need of two sets of instances. For example, suppose in a village by using a particular fertilizer people get good harvest and in the neighboring village, people have not used that fertilizer and have not got good harvest then the causal j connection between two incidents namely using the fertilizer and getting good harvest can be suspected.
Thus where the presence of one incident leads to the occurrence of another incident and the absence of that incident leads to the absence of the other incident a causal connection between the two phenomena can be suggested.
Because of the double agreement i.e. agreement in presence and the agreement in absence the causal connection can be ascertained with higher probability. The following example will illustrate it.
Symbolical example: – Positive Set
ABC are followed by abc
A B D are followed by abd
A C D are followed by acd
AND Negative Set
B C D are followed by bcd
C D E are followed by cde
B E F are followed by bef
\ A and a are causally related.
In this example among the positive instances A is common to all the three antecedent instances and correspondingly as is common in the three consequent instance.-, Moreover, among the negative instances A is absent in the three antecedent instances and correspondingly a is absent in the consequent instances. Here a causal connection between A and a is accepted.
This method is based on both the first and second principle of elimination. It follows that whatever is absent in presence of a phenomenon cannot be the cause of that phenomenon and a circumstance in whose presence the effect does not occur cannot be the cause. Basing on these principles. B, C, M, G, E, F, D cannot be the cause.
1. In the cricket matches where Saurav is captain of the Indian team, the team wins and when Saurav is absent the term loses. Other circumstances being the same the captainship of Saurav is the cause of the winning of the team.
2. The students who follow the textbooks regularly get good marks. The students who do not follow the textbooks but follow notebooks do not get good marks.
Other conditions remaining almost the same it follows that following textbooks and securing good marks are causally connected.
1. In the joint method there is application of the method of agreement and the method of difference. Hence the principles of elimination of both these methods are applied in this method to eliminate the accidental factors and to find out the real cause.
Whatever antecedent can be left out without affecting the effect cannot be the cause. Similarly what cannot be eliminated without affecting the phenomenon must be causally related with it.
2. It is not a primary method. It is the modification of the method of agreement and the method of difference. Fowler calls this method “the double method of agreement” and Bain calls it “the method of double agreement”.
But Mill considers this method as a special modification of the method of difference and calls it, “the indirect method of difference”. Venn considers this method as the method of exclusion.
3. In the method of difference only two instances are taken into consideration whereas in the joint method two sets of instances are taken into consideration.
The method of difference strictly adopts experiment for finding out the instances but in the joint method instances may be collected either by experiment or by observation.
4. The joint method proves the causal relation in an indirect way. The method of agreement suggests a causal relation through the positive set and the joint method confirms the suggestion through another set, the negative set. Hence it is supposed to be the method of proof.
Advantages or merits of the method:
1. It is a dependable method because it proves the supposition obtained through the method of agreement. The method becomes more dependable if the instances are chosen carefully.
2. This method can be applied to the cases of observation and also to the cases of experiment. When the instances are taken from observations it becomes a modification of the method of agreement and when the experimental instances are taken into consideration it becomes a modification of the method of difference.
3. It has a wide range of application. The cases of plurality of causes cannot create difficulty for this method. For it considers the negative instances that help to avoid the unconnected common antecedent.
1. Mill claims that this method eliminations the difficulties arising out of the plurality of causes. The method of agreement is unable to tackle it.
Some logicians point out that the possibility of plurality of causes is not agreeably eliminated by this method. Because the particular agreeable factor may be the cause so far. But there may remain many other cases unexamined where the cause may be different. So this method does not fare better to tackle the problem arising out of plurality of causes.
2. This method also fails in case of intermixture of effects. The method can be applied when the antecedents and the unsequents are distinguishable in conjunction of causes and intermixture of effects. But if the consequents make a complex phenomenon it is not possible to apply the joint method.
3. The joint method is of no value in case of permanent causes. For it is not possible to procure negative instances.
4. It also fails to distinguish the cause from co-existence. Because in respect of the co-existence of incidents two phenomena may be present or absent in several instances. But they may not be causally related.
Lightning always precedes thunder and they are also absent together. But lightning is not the cause of thunder, but they are the co-effects of some other cause.