Concept of Probability and Esp

SIR,

In connection with the recent discussion on Extra-Sensory Perception (ESP) I would like to draw attention to some aspects of the concept of probability, for it is indeed on this concept that the believers in ESP rest their claims. In this communication, neither the data nor their interpretation are of primary interest to me.

Let me at once grant the proponents of ESP their main point; the evidence they present cannot be explained by chance. Then what? What meaning can we attach to the statement that the findings are due to non-chance? The matter is not as simple as the proponents of ESP would have us believe. They imply that since the evidence cannot be explained as non-chance, it must be explained on the basis of some lawful causality. This however is a basic error in logic.

The thinking of P. W. Bridgman on this question is of singular interest to all those concerned with ESP.1 I will quote the essential points he makes.

'We have to ask what we mean when we say "this event was not chance." Since we have already made an attempt to tell what we mean when we say "this event was chance", we might be tempted to think that our new question is trivial and that its answer is implied in the answer we have already given. I think, however, and this is perhaps the crux of this note, that this is by no means the case. "Was chance" and "was not chance" are not simply related to each other as two terms in traditional Aristotelian logic, subject to the rule of the excluded middle. …

There is a deep-seated difference between the way in which positive and negative probability arguments are fruitfully applied in practice to concrete situations, to which we have seen that the concept of probability does not rigorously apply at all. If the situation is a positive one, which we can characterize by saying "Here we have the play of chance", then we can draw fruitful conclusions from the mere statement, without going further. This is shown by countless examples, as in the ta