Title: FAC! (Fallacy of Affirming the Consequent)
Course: UGQuants (SSPS) > Section: Causality

FAC! (Fallacy of Affirming the Consequent)

In this module you have seen how important it is to consider control variables carefully when trying to assess causality. When we are dealing with non-experimental data (the situation we usually face), we can never obtain direct evidence about the causal processes we are investigating. Rather we obtain evidence that is consistent with the causal theory we have suggested. The models we construct can never take all possible factors into account . It is important to keep this in mind, because our evidence may also be consistent with other theories – that is,  it may only be part of all the evidence that is relevant to the understanding of the relationship. To be accurate, all we can say is that we have not disproved our theory. To claim more certainty for our knowledge would be to fall into a trap that philosophers call the fallacy of affirming the consequent, or FAC, for short.

For example. I might arrange to meet my friends at nine o'clock Sunday evening in a café we all like. But when I go, none of them are there. What might I conclude from this 'experiment'? The logic might go something like this

True friends keep appointments (or at least let you know if they cannot).

My friends did not keep the appointment.

Therefore they cannot be true friends.

When the variable friend takes the value 'true', the variable appointment takes the value 'keep'. In this 'experiment' the variable appointment takes the value 'not keep', so the variable friend cannot take the value 'true'. Have I proven that my friends are not true friends?

The logical problem is that although my theory of friendship betrayal is consistent with events, the events cannot prove that my theory is accurate. To assert otherwise would constitute a case of the fallacy of affirming the consequent: that event B happened because event or condition A occurred. This logic is false, as there may also be other, equally plausible, explanations of what occurred. Maybe the change to summertime meant that the clocks had gone forward on Saturday night without me realising. Maybe I, or they, made a mistake about which café we agreed to meet in. Maybe their bus broke down ...

The key point is this: we can readily discount some theories when our empirical evidence contradicts them. However, when our empirical evidence is consistent with our theory, we still need to remain open to other possible, equally consistent, theories. We can then elaborate our knowledge by devising tests to choose between these competing theories. Theory elaboration can thus proceed in close step with empirical research, with new research refining the theory, or even pointing it in a new direction.