Nevertheless, this book was seen by Mme. Flammarion, who was looking behind the curtain; and it disappeared from the eyes of the persons who were in front, notably M. Baschet, M. Brisson, M. J. Bois, Mme. Fourton and myself. We were not expecting this miracle in any way; we were stupefied by it; we asked what had become of the book, and it seemed as if it had fallen behind the curtain. Collective hallucination? But we were all in cool blood, entirely self-possessed. If Eusapia had been able to adroitly slip her hand around and seize the book through the portiere, the bare outline of the book would not have been seen, but a protuberance of the portiere. How great a value the sight of this thing passing through a portiere would have as a scientific datum, if one were only sure of the absolute honesty of the medium,--if, indeed, this medium were a man of science, a physicist, a chemist, an astronomer, whose scientific integrity would be above suspicion! The mere fact of the possibility of fraud takes away ninety-nine one-hundredths of the worth of the observation, and makes it necessary for us to see it a hundred times before being sure. The conditions of certainty ought to be understood by all investigators, and it is curious to hear intelligent persons express surprise at our doubts, and at the strict scientific obligation we are under to lay down these conditions. In order to be sure of abnormalities like these levitations, for example, we must make sure of them a hundred times over; not see them once, but a hundred times. It seems to us impossible that matter could pass through matter. You place for example a stone upon a napkin. If one should tell you that he has found it under the napkin, without any break in the continuity of the tissue, you would not believe him. However, I take a piece of ice, weighing say two pounds, and place it upon a napkin; I place both upon a strainer, in the oven; the piece of ice melts, passes through the napkin, and falls drop by drop into a basin. I put the whole thing into a freezing machine, the melted water congeals again; the piece of ice weighing two pounds has passed through the napkin. It is very simple, you think. Yes, it is simple because we understand it. But, of course, this is not the same case as that of the book. Yet, after all, it is matter passing through matter, after a transformation of its physical condition. We might seek explanations, invoke the hypotheses of the fourth dimension, or discuss the non-Euclidian geometry. It seems to me more simple, however, to think that, on the one hand, these experiments are not yet sufficient for us to make an absolute affirmation, and that, on the other hand, our ignorance of everything is formidable and forbids us to deny anything.