In arguing from effect to cause the adequacy of the alleged cause must be proved, the fact that no other cause intervened between the alleged cause and the observed effect must be clearly demonstrated, and the circumstances of the case must show that the alleged cause was not prevented from operating. With these requirements fulfilled such an argument may be regarded as sound. It will be seen that the application of these rules requires sound judgment and practical common sense. The argument will be effective in persuading others only when every requirement is met in a plain, straightforward manner.
II. Argument from cause to effect.
The argument from cause to effect is one which relies upon an observed cause to prove or foretell the existence of an unobserved effect. For example, I observe that the temperature is very low; the thermometer registers below zero and the exposed parts of my body tingle with cold when I am out of doors. This is a cause of several effects. One of them is that the pond near my home will be frozen over. I observe the cause (i. e., the low temperature) and at once state the effect (i. e., the ice on the pond). The process by which I reached this conclusion is called a priori reasoning. The conclusion is based upon circumstances observed before the disputed fact. Likewise, I observe that it is now beginning to rain and that appearances indicate a heavy downpour. I at once come to the conclusion that the path across the meadow will be muddy when I pass over it in half an hour from now.
This case differs from the preceding one only in the fact that in the first case the effect existed when the cause was observed, whereas in the latter case the effect did not exist when the cause was observed. In both cases the observed cause is the basis for determining the unobserved effect. In this way we may reason from the past to the present, from the remote past to the less remote past, from the present to the future, from the near future to the more remote future, or from the past to the future.
The student will doubtless have observed that the argument from cause to effect as well as that from effect to cause is a special form of deduction. The syllogistic form may be applied to either of these processes of reasoning for the purpose of testing their strength. Applying the syllogistic form to the a priori reasoning involved in one of the preceding illustrations we have:
A. Low temperature is always followed by the forming of ice.
B. This is low temperature.
C. Therefore it is followed by the forming of ice.
Applying the syllogistic form to the a posteriori reasoning involved in one of the examples given under the discussion of that process we have:
A. All times when the ground is covered with snow are times when there has been a snowfall.