Therefore, I have given my preferences to the study of physical phenomena, because in such I have not to consider the mental condition of the subject, nor have I any of those delicate analyses to make, the complexity of which I have just mentioned. I have to defend myself against only two enemies, the fraud of others and my own illusions. Now, I feel certain of never having been the victim of either. When, for example, in the refreshment-room of a railway-station, in a restaurant, in a tea-shop, I have observed, in broad daylight, a piece of furniture change place of its own accord, I have a right to think I am not in the presence of furniture especially arranged to produce such effects. When the unforeseen nature of the experiment excludes the hypothesis of preparation, when, by sight and touch, I make sure of the absence of contact between the experimenters and the article which is displaced, I have sufficient reasons for excluding the hypothesis of fraud. When I measure the distance between the objects before and after the displacement, I have also sufficient reason for excluding the hypothesis of the illusion of my senses. If this right be refused me, I should really like to know how any fact whatever can be observed. No one is more convinced than myself of the frailty of our impressions and the relativity of our perceptions; nevertheless, there must be some way of perceiving a phenomenon in order to submit it to impartial observation. Besides, the supposed reproach of illusion cannot be applied in a general sense; to admit its justice would be to do away with the very foundations of our sciences. It can only be applied to me as an individual, and I willingly admit that it is impossible for me to exculpate myself. In vain might I plead that I am persuaded of the regularity of my perceptions, in vain assert that I observe no tendency to illusion in myself, my testimony would remain none the less suspected. Consequently, I have but one reply for those who mistrust my qualifications as an observer, and that is to invite them to take the trouble of experimenting on their own account, using the methods which I have adopted. If, _a priori_, they wish to lay down their own conditions, they run the risk of receiving no appreciable results. When they have obtained a few plain facts they will be able to vary the conditions of experimentation, and satisfy the legitimate exigencies of their own reason. That is what I did, and if I cannot solemnly affirm the reality of the phenomena which I have observed, I can at all events affirm my personal conviction of their existence. Maybe I am showing an exaggerated mistrust of myself by thus only affirming my subjective conviction, and in not venturing to affirm with a like energy the objective reality of the things I have seen. Yet I trust no one will blame me for my prudent reserve. What man can say he has never made a mistake?