“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”
Mill has considered the method of difference to be the ‘method par excellence’. But Mill’s opinion regarding the method of difference may be considered as mistaken if we compare that method with the method of concomitant variation.
In case of the method of concomitant variation there is variation of one factor in respect of degree or quantity. This variation in respect of a single factor in a number of cases leads to some authentic causal relation. Hence this method may be graded as the method par excellence.
Symbolical Example:
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(i) A 1 B C are followed by x1 y z
A2BC are followed by x2 y z
A3 B C are followed by x3 y z
\ A is the cause of x
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(ii) A1 B C are followed by x1 y z
A2CD are followed by x2 z w
A3 E F are followed by x3 p q
\ A and x are casually related
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In the above examples A varies from instance to instance in the antecedent side and correspondingly x also varies from instance to instance in the consequent side.
In the symbolic example (i), while the accompanying circumstances remain the same, BC in the antecedent side and yz in the consequent side, in the symbolic example, (ii) the accompanying characteristics change from instance to instance.
But in both the examples when the antecedent ‘A’ quantitatively changes corresponding changes occur in the consequent ‘x’ whether the accompanying circumstances remain the same or not.
The variation between A and x suggests a possible causal relation between them. Thus depending upon this single variation a causal connection between A and x is established.
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Here the third principle of elimination is followed. That is “an antecedent and a consequent rising and falling together in numerical concomitance are to be held as cause and effect”.
It amounts to saying that when the causal factor varies in any manner the effect factor also varies in some particular manner. The cause and effect vary together with a definite proportion.
Concrete example
1. When the rate of gold increases it is also seen that the sale of gold decreases. This shows that the increase of rate and the decrease in the sale are causally related.
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2. When there are increases of temperature in the body there is also increase of mercury level in the thermometer. This variation shows that heat and increase of the mercury level (expansion of mercury) are causally connected.
3. The variable relation between the size and position of the moon and the rise and fall of tides in the sea is shown as the example of this method my Mill.
When there is increase of the size of the moon on the sky the rise of tide increases in the sea. When there is decrease of the size of the moon, the rise of tides falls. Thus the two factors are causally related.
Characteristics
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1. The method of concomitant variation is not a primary method for it is either a modification of the method of agreement or a modification of the method of difference. When the accompanying circumstances are the same in all the instances, it is a modification of the method of difference.
On the other hand when the accompanying circumstances are different in the instances, the method is a modification of the method of agreement.
2. This method is founded on the quantitative aspect of carnation. The cases of quantitative variations can be taken into account for deciding the causal relation. Sometimes the method is called a statistical method as it can be expressed graphically.
3. It is primarily a method of discovery but it is also quite helpful in case of scientific investigations. So it is both a method of observation and experiment.
When the method is a modification of the method of agreement, it is based on observation. When the method is a modification of the method of difference, it is based on experiment.
Advantages:
1. The significant advantage of this method is that in case of inseparable homogenous intermixture of effects this method is very much helpful. By the application of quantitative variation to any particular factor the causal connection can be determined.
2. This method is also helpful in case of permanent causes. The permanent causes cannot be dropped. But they can be increased or decreased. In this way the principle of variation is applicable to it. This method is specially applicable in case of permanent causes.
3. The result obtained by this method is highly reliable especially when it is applied experimentally.
Disadvantages:
1. It is a quantitative method. It is not applicable in case of qualitative variations.
2. This method is applicable in case of quantitative variations, but up to a certain limit. The quantitative variation cannot be applied after a particular limit.
3. A proportional variation may also be possible in the instances of co-existences. In such cases the method is unable to distinguish between the cause and co-existence. We have experienced proportional variation of time span of the day and night. It is an instance of co-existence. But one may confuse it as a causal instance attaching emphasis on the factor of variation.
4. A condition may vary proportionately. In such case this method is unable to distinguish between the cause and the condition.
5. This method is not applicable in case of plurality of causes. Because this method relies only on a case of single variation.