THE PRECAUTIONS described in the last chapter are those necessary for the
experimenter to satisfy himself that his experiments, if successful, are
indicating the operation of paranormal capacities on the part of his
experimental subject. They are not in principle different from the kind of
precautions that would be taken in any other branch of experimental science, as
for example, in an experiment on visual acuity in a psychological laboratory.
Let us call such precautions primary precautions. If full primary precautions
are taken by an experimenter and if also his experimental subject is succeeding
too often for his successes to be reasonably attributed to chance, then the
experimenter can be satisfied that the experimental subject has whatever
paranormal capacity (ESP, precognition, or PK) the design of the experiment was
adequate to reveal.
The experimenter can be so satisfied but not necessarily anyone else. All that
is normally required in an experiment is that its conditions should be such as
to satisfy the experimenter, and for most experiments in parapsychology, the
primary precautions are enough. They are not, however, enough if the
experimenter has set himself the task of producing evidence that will be
completely convincing to other people. These others may have varying degrees of
unwillingness to believe in any results which seem to favour a paranormal
hypothesis. Some of them may argue that ESP is impossible and that dishonesty on
the part of the experimenter (or of one or more of his helpers) may be admitted
to be very improbable but is plainly not impossible. So being faced with a
choice between an impossible and an improbable explanation, we must choose the
improbable one. In any experiment with full primary precautions the
experimenter can know that he has not cheated; he may try to devise a system of
secondary precautions so that others can know it too.
 George R. Price, 'Science and the Supernatural',
Science, 26 August 1955 (abstracted in the Journal of Parapsychology,
XIX (Dec. 1955), pp. 238-41).
 This is, of course, a special type of experiment; most parapsychological
experiments have some other object than that of convincing other people that the
experimenter did not fake his results. It is a common misunderstanding of the
situation to suppose that all experiments should conform to the design dictated
by this special requirement and that those which do not are therefore imperfect.
If we require coercive proof of the reality of ESP by means fully safeguarded
against the possibility of fraud by the experimenter (or by any other person
helping to conduct the experiment), this requirement would be met by only one
experiment that had been so safeguarded and had been successful. If there is
already more than one experiment that has been so safeguarded, it would be a
waste of time to do any more of the same kind. On the other hand, if all
existing experiments fall short of perfect safeguarding against experimenters'
fraud because they all can be shown to have imperfections (ways in which,
whether he did or not, the experimenter could have cheated), and if improved
designs of experiment would get rid of those imperfections, then it would seem
that the sensible thing would be to carry out those new experiments with
It may be that neither of these views about the experimental situation is the
true one. My own opinion is different from both; it is that a completely
conclusive experiment of this type is, in principle, impossible. I do not think
this impossibility is peculiar to parapsychology: it would apply equally to an
experiment in any other field in which the untrustworthiness of the experimenter
was seriously considered as an explanation of results. It does not trouble us in
other fields because generally their experiments are more easily repeatable than
those of parapsychology. It would be an exaggeration to say that
parapsychological experiments are not repeatable; they are constantly being
repeated. There is no certainty that a new investigator repeating an
investigation will get the same result but a considerable probability that he
will. If the strength of experimental evidence for the reality of paranormal
events is to be increased, there is better hope of this end being achieved by
the multiplication of successful experiments with adequate primary precautions
than by the pursuit of the single experiment with a water-tight system of
precautions against experimenters' fraud.
Even if this be accepted, it remains an interesting question whether the
experiments that have already been done with secondary precautions against fraud
by the experimenter or his colleagues are as good as they can be made or whether
new experiments of this type should be designed with new and better precautions.
Whatever we may think of their finality, they remain an important part of the
case for the reality of ESP. It remains a strong argument for the reality of
paranormal phenomena that various experiments force as to choose between
acceptance of their reality and the very unlikely alternative of a conspiracy to
deceive entered into by a number of persons who seem honest and reliable. This
argument remains strong even if it is not absolutely coercive against those who
accept as axiomatic the proposition that paranormal phenomena are impossible.
In experiments with this aim, there have been various ways adopted for
controlling the experimenter. The record that the experimenter makes of the
target series of cards may be handed to a third person or group of persons
before the experimenter knows what the percipient has guessed. This same person
or group of persons also may receive the guessed series before the percipient
knows what the target series was, and the comparison between the two series may
be made by this third person or group of persons. Then if adequate precautions
have been taken to see that the guesses are those which were made by the
percipient and that the target series represents the actual order in which the
cards were presented by the agent, and if also an independent witness has
shuffled the target set of cards and has watched to see that the cards so
shuffled are the ones used in the experiment, then one has ensured that
successful results could not have been fraudulently produced by the experimenter
himself. It would still be possible that they could be fraudulently produced by
the experimenter in collusion with all or with some members of the witnessing
group. This possibility can be made negligibly small by increasing the
reliability of the witnesses and the number of reliable witnesses who have had
complete control for some part of the experimental series. But the possibility
does not become zero; it is still reasonable for the critic who rejects the
possibility of ESP to say that if no other normal explanation remains open,
there must have been collusive fraud to produce the results.
Instead of having records of targets and guesses delivered to some person or
group outside the experiment, the same object (of making it impossible for the
experimenter to cheat) may be attempted by having the recording of targets and
guesses and the process of comparing them witnessed throughout by outside
witnesses. These may be different persons on different occasions and if success
is repeated on different occasions, it becomes difficult to suppose that all
these witnesses have joined a conspiracy to mislead the public. Again the critic
may say: very difficult but not impossible, and what the experiments seem to
prove are impossibilities.
In the early days of experimenting in ESP, it was not felt necessary to take
elaborate precautions against the experimenters cheating. Many experiments with
this aim have, however, now been carried out; I will mention three which are
typical and are amongst those most discussed. The earliest was the Pearce-Pratt
series of experiments carried out at Duke University in 1933-4. Here Dr Pratt
was the agent, while a divinity student called H. E. Pearce was the percipient.
There were four sets of experiments totalling 74 runs through the pack. Since
there were twenty-five cards in each pack, the total number of the subject's
guesses was 1,850, of which one would expect 1/5th (that is 370) to be right by
chance. The number of right guesses actually scored was 558 which is about 50
per cent more than the 370 to be expected by chance. The percipient was sent by
himself to the library and the agent was in another building, 100 yards away in
some experiments and 250 yards in others. The precaution taken against cheating
by percipient or agent or by both in collusion was that the records of targets
and of guesses were handed in a sealed packet to Dr
Rhine immediately after the
experiment before they were compared. This precaution does not, of course,
eliminate the theoretical possibility that Pratt, Pearce, and Rhine might be in
a joint conspiracy to deceive as to the results. It has also been suggested by
Hansell that the percipient might himself have produced the results dishonestly
if he had left the library after he was seen to enter it and had managed to
conceal himself somewhere where he could see the laying out of the target packs.
It is obviously highly improbable that he would have tried to do this, and also
that he could have succeeded in doing it without detection. Highly improbable
but not impossible, so this series of experiments will not be sufficiently
convincing to a critic who holds that the ESP explanation is impossible.
 J. B. Rhine and J. G. Pratt, 'A
Review of the Pearce-Pratt Distance Series of ESP Tests', Journal of
Parapsychology, XVIII (1954), pp. 156-77.
 C. E. M. Hansell, 'A Critical Analyses of the Pearce-Pratt Experiment',
Journal of Parapsychology, XXV (1961), pp. 87-91.
Later at Duke University came the Pratt-Woodruff experiment. Here also there
were safeguards against the experimenter cheating, and this series also has been
subjected to criticism.
 J. G. Pratt and J. L. Woodruff, 'Size of
Stimulus Symbols in Extra-Sensory Perception', Journal of Parapsychology,
III (1939), pp. 121-58.
 C. E. Hansell, 'A Critical Analysis of the Pratt-Woodruff Experiment',
Journal of Parapsychology, XXV (1961), pp. 99-113.
The most elaborately safeguarded experiment to date has been that of Dr
Mrs Goldney carried out with the gifted percipient Shackleton in London in
1941. These experimenters used pre-arranged lists of the digits 1-5 prepared
from a table of random numbers. There were five target cards with animal
pictures on them, one of which was exposed to the agent, in accordance with a
code for translating the number on the list to one of the five animal
representations. It was these animals (lion, elephant, etc) that Shackleton
guessed. Various outside witnesses were brought in for each experiment to see
that the experimenters were neither deliberately falsifying their results nor
doing so accidentally by systematic recording errors. An earlier pilot
experiment had shown that Shackleton seemed to be guessing, not the card turned
up, but the card ahead of that, and the experiment was designed to discover
whether this guessing one ahead would continue. Over a long series, he showed a
very high rate of success, varying about eight right guesses per pack instead of
the chance expectation of five. The rate of success was such that it could not
reasonably be attributed to chance, so the critic who believes ESP to be
impossible must consider the alternative hypothesis of deliberate fraud.
 K. M. Goldney and S. G. Soal, 'Experiments in
Precognitive Telepathy', Proceedings of the SPR, XLVII (1943), pp.21-150.
In 1955, Dr G. R. Price of the University of Minnesota suggested that the
Soal-Goldney results could have been produced by fraud if a sufficient number of
the witnesses had conspired with the experimenter to produce the deception [see
ref. ]. Later it was suggested by Hansell that the successful agents
might have memorized the card order and produced successful results by changing
the coding cards so as to produce maximum scores.
 C. E. M. Hansell, 'A Critical Review of the
Experiments with Mr Basil Shackleton and Mrs Gloria Stewart as Sensitives',
Proceedings of the SPR, LII (1960), pp. 1-42.
These are perfectly proper criticisms; they are not suggestions that anyone has
cheated but assertions that the conditions of the experiment were such as not to
preclude the possibility of some individual or group of individuals having
produced the results by fraud. The particular suggestions as to how fraud might
have produced the results may be erroneous, for example, it has been objected
that Hansell's suggestion could not account for as high a level of success as
was obtained in the Soal-Goldney experiment. If this is admitted, the essentials
of the criticism remain. 'The impossible does not happen. ESP is impossible.
Therefore, ESP does not happen, and if any experimental results are such that
they can only be explained by ESP or by fraud, the explanation by fraud must be
accepted, however improbable it may appear.' The critic is naturally happier if
he can suggest how the fraud took place, but it is not essential. If the above
argument is accepted, fraud must have taken place, either in the way suggested
or in some other way.
Obviously many people do not accept the argument. They will not deny the first
premise, that the impossible does not happen. This indeed is a tautology which
it would be nonsensical to deny. The second premise that ESP is impossible is
not self-evident and may reasonably be doubted. We may indeed consider that our
intuitive opinions as to what is and what is not possible are so unreliable that
the only way to make sure whether any kind of event is possible is to find out
empirically whether or not it happens. If it happens, it must be possible,
whatever our intuitions may be in the matter; if it cannot be shown to happen
then it may be impossible.
But for those who do accept as axiomatic the impossibility of ESP, the
conviction that ESP does not happen cannot be overthrown by multiplication or
refinement of the system of secondary precautions. Somewhat inconsistently Price
suggested that a new ESP experiment should be done with the cards in welded
steel containers and a jury of twelve strongly sceptical scientists. But even if
such an experiment were successful, the critic who accepted the impossibility of
ESP would still have to say that the twelve scientists must have entered into a
conspiracy to defraud by some method unknown. The improbability would be great
but it would still not be impossible. Even then we should have to prefer an
improbability to an impossibility.
This being the case, we must consider whether there is not another road to
certain conviction. Undoubtedly there is, and a good example of a sceptical
psychologist taking that road, is to be found in the experiment of Professor
Lucien Warner reported in 1937. The one person who knows whether the
experimenter has cheated is the experimenter himself. So if he carries out a
successful experiment with full primary precautions and knows that he has not
himself cheated, the evidence will be conclusive to him. A successful experiment
means, of course, one too successful to be reasonably attributed to chance, and
full primary precautions means full precautions against the various sources of
error (sensory leakage, recording errors, etc.) and against the possibility of
the subject or any other participant in the experiment producing the results
fraudulently. Professor Warner used a percipient who had already shown his
ability to succeed in ESP tests. He had the percipient in a locked room on a
different floor from the experimenter with communication only by signal from
percipient to experimenter and not the other way. In 250 guesses he got 93 hits,
an excess of 43 over the 50 right guesses that would be expected if there were
only chance correspondence between the target cards and the percipient's
guesses. The odds against such an excess occurring by chance are of the order of
a thousand million to one, so the result is convincing evidence of the
percipient's guesses being determined by the target cards, and therefore of the
reality of ESP; convincing evidence for Professor Warner himself and for anyone
else who believes in Professor Warner's good faith. Not, of course, for the
critic who believes that ESP is more unlikely than any alternative explanation;
if he is to be convinced, it must be by doing a similar experiment himself.
 Lucien Warner, 'A Test Case', Journal of
Parapsychology, I (1937), pp. 234-8.
It might be a good thing if parapsychological research laboratories sometimes
refrained from the intensive experimenting with a successful percipient which
seems to inhibit their ESP capacity. Instead some might remain as a pool of
potential experimental subjects who could be made available for short series of
experiments by sceptical critics of ESP who wished to expose themselves to the
risk of being convinced by the only means conclusive against the alternative
explanation of experimenter's fraud.
It may be that convincing those who deny the possibility of ESP is less
important than is commonly supposed. If, however, the attempt is made, this
seems a more promising way than the elaboration of secondary precautions. These
are open to the objection of leading to unnecessary complications of design in
experiment, and complications lead to new loop-holes. It is significant that
Hansell's criticisms of the Soal-Goldney experiment suggest as the possible
means of fraud a feature (the five cards used for coding) which was introduced
to make it more difficult for the experimenters to cheat. New complications
might be introduced to eliminate these possibilities but a new critic might come
along to show how these have introduced new possibilities of error. I think it
is better to stick to simple experimental designs, and to eliminate the
theoretical possibility of experimenters cheating, not by devising elaborate
precautions against this possibility, but by providing favourable opportunities
for critics to see whether they can, as experimenters, repeat the reported
"Experimental Psychical Research" by Robert
Thouless (Middlesex, Baltimore, Victoria: Penguin Books, 1963).