On the contrary, the muscular sense is one of the most unfaithful. Unless one has experimented oneself, it is impossible to imagine how frequent unconscious and involuntary movements are. These movements are of very feeble amplitude; they are slight, but they end by acquiring a certain amount of force. It will then be noticed that the assistants accuse each other reciprocally of pushing the table, and it is not rare to see angry discussions arise on these occasions. This is a frequent fact of observation. I have also very frequently noticed tactile hallucinations with impressionable experimenters, who easily imagine diverse contacts. The sense of smell sometimes perceives imaginary odours, but it is somewhat rare. I have not observed any hallucinations of taste. Another cause of error which requires pointing out is fatigue on the part of the experimenters. Every phenomenon which is produced after a long period of waiting stands many chances of being badly observed. The attention kept for a long time on the _qui vive_ becomes weary, gives place to abstraction, and often the phenomena takes the experimenters by surprise; hence they are unable to examine the conditions with certitude. It is also bad to hold very long seances, fatigue quickly setting in. Such are the principal causes of positive errors; that is to say, of errors tending to persuade one of the existence of an imaginary fact; negative errors, that is to say, those which tend to make one look upon a real fact as an imaginary one, are not less dangerous than positive errors. In the first place, _parti pris_ is to be pointed out. If we wish to experiment with success, we must experiment without credulity, without faith, even without confidence; but we must not be determined only to meet with fraud. We must not experiment naïvely. If, at the beginning of a seance, it be useful to allow freedom in order to put the force _en train_, as Ochorowicz wisely recommends, once the phenomena are established, we must control them with the greatest care. But we should make our intentions known to the medium and to the personification. This, I think, is an indispensable precaution. The personification will always consent to it; but this does not mean we will always obtain the wished-for result. We must not allow the medium or the personification to think we are their dupes if they fraud; we must tell them, gently but clearly, that they are not giving anything good. Equivocation is to be carefully avoided, all misunderstanding is to be shunned.