Experience must be our guide. To shut the door on actual observation and experiment in this particular region, because of preconceived ideas and obstinate prejudices, is an attitude common enough, even among scientific men; but it is an attitude markedly unscientific. Certain people have decided that inquiry into the activities of discarnate mind is futile; some few consider it impious; many, perhaps wisely mistrusting their own powers, shrink from entering on such an inquiry. But if there are any facts to be ascertained, it must be the duty of some volunteers to try to ascertain them: and for people having any acquaintance with scientific history to shut their eyes to facts when definitely announced, and to forbid investigation or report concerning them on pain of ostracism,--is to imitate a bygone theological attitude in a spirit of unintended flattery--a flattery which from every point of view is eccentric; and likewise to display an extraordinary lack of humour. ON THE POSSIBILITY OF PROGNOSTICATION I do not wish to complicate the issue at present by introducing the idea of prognostication or prevision, for I do not understand how anticipation of the future is possible. It is only known to be possible by one of two processes-- (_a_) Inference--_i.e._ deduction from a wide knowledge of the present; (_b_) Planning--_i.e._ the carrying out of a prearranged scheme. And these methods must be pressed to the utmost before admitting any other hypothesis. As to the possibility of prevision in general, I do not dogmatise, nor have I a theory wherewith to explain every instance; but I keep an open mind and try to collate and contemplate the facts. Scientific prediction is familiar enough; science is always either historic or prophetic (as Dr. Schuster said at Manchester in the British Association Address for 1915), "and history is only prophecy pursued in the negative direction." This thesis is worth illustrating:--That Eclipses can be calculated forwards or backwards is well known. A tide-calculating machine, again, which is used to churn out tidal detail in advance by turning a handle, could be as easily run backwards and give past tides if they were wanted; but always on the assumption that no catastrophe, no unforeseen contingency, nothing outside the limits of the data, occurs to interfere with the placid course of phenomena. There must be no dredging or harbour bar operations, for instance, if the tide machine is to be depended on. Free-will is not allowed for, in Astronomy or Physics; nor any interference by living agents.