Practical Argumentation · Chapter 152. Argument from Sign
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ARGUMENT FROM EFFECT TO CAUSE. The process of reasoning from effect to cause is called argument from sign. Since every circumstance must be the result of some preceding circumstance, the arguer tries to find the cause of some fact that is known to exist, and thereby to establish the existence of a hitherto unknown fact. For instance, when one sees a pond frozen over, he is likely to reason back to the cause of this condition and decide that there has been a fall in temperature, a fact that he may not have known before. The sight of smoke indicates the presence of fire. Human footprints in the snow are undoubted proof that someone has been present.
In the following quotation, the recent prohibition movement in the South is said to be a sign that the voters wish to keep liquor away from the negro:—
What is the cause of this drift toward prohibition in the South? The obvious cause, and the one most often given in explanation, is the presence of the negro. It is said that the vote for prohibition in the South represents exactly the same reasoning which excludes liquor from Indian reservations, shuts it out by international agreement from the islands of the Pacific, and excludes it from great areas in Africa under the British flag; and that, wherever there is an undeveloped race, the reasons for restrictions upon the liquor traffic become convincing. [Footnote: Atlantic Monthly, May, 1908, p. 632.]
The strength of this kind of reasoning depends upon the closeness of the connection between the effect and the assigned cause. In testing argument from sign, one should ask:—
(1) Is the cause assigned adequate to produce the observed effect?
This test is precisely the same as the test of adequacy for antecedent probability. One could not maintain that the productiveness of a certain piece of ground was due entirely to the kind of fertilizer used on it, nor that a national financial upheaval was caused by the failure of a single unimportant bank. In each of these cases the cause suggested may have assisted in producing the result, but obviously it was not of itself adequate to be the sole cause.
(2) Could the observed effect have resulted from any other cause than the one assigned?
If several possible causes exist, then it is necessary to consider them all, and show that all the causes except the assigned cause did not produce the observed effect. If an employer who has been robbed discovers that one of his clerks has suddenly come into possession of a large sum of money, he may surmise that his clerk is a thief. This argument is valueless, however, unless he can show that his employee did not receive his newly acquired wealth through inheritance, fortunate investment, or some other reasonable method. But if no other reason than burglary or embezzlement can explain the presence of this money, the argument is very strong.
One might greatly weaken the argument (quoted earlier) which assigned the cause of the recent prohibition movement in the South to the presence of the negro by showing that this action was not the result of the assigned cause, but largely of another cause. He might prove that during the debate in the Georgia Legislature upon the pending prohibitory bill, the negro was not once mentioned as a reason for the enactment of prohibition; and that the chief arguments in favor of prohibition were based upon the fact that the saloon element had formed a political ring in the South and were controlling the election of sheriffs, mayors, aldermen, and legislators.
ARGUMENT FROM EFFECT TO EFFECT. Argument from sign also includes the process of reasoning from effect to effect through a common cause. This method consists of combining the process just described with the argument from antecedent probability. A reduction of wages in one cotton mill is a sign that there may be a reduction in other cotton mills. Here the reasoning goes from effect to effect, passing, however, though perhaps the reasoner is not aware that the process is so complex, through a cause common to both effects. In full, the reasoning would be: a reduction in the first mill is the result of the cause "hard times"; it is then antecedently probable that this cause will produce a similar reduction of wages in other mills.
This method may be represented by the following figure:—
Cause / \ / \ / \ / \ Effect Effect
Only one effect is known; the other effect is inferred, first, by a process of reasoning from a known effect to an unknown cause, and secondly, by the process of reasoning from this assumed cause to an unknown effect.
This method of reasoning is sound and legitimate when both effects have the same cause. Its weakness lies in the fact that it may be attacked on two sides: on the reasoning from effect to cause, and on the reasoning from cause to effect. If the connection can be broken in either process, the argument is overthrown. The tests to be used have already been given.