Consider two fairly simple, natural occurrences: a storm at sea and an earthquake. Many descriptions of these things have been made, but what interests us here are their scientific descriptions. In the first case, then, we have to deal with changes in the pressure and temperature of the atmosphere, with violent motions of the air and with waves in the sea. In the second case we deal with stratified rocks that are in a state of severe strain. When this strain exceeds a certain limit the rocks are fractured and waves are set up in the elastic materials of the earth's crust. In most cases storms and earthquakes happen when we do not expect them, and so there is usually little time to investigate them in an adequate manner. But there is no doubt whatever that if we could make all the researches that are practicable we could “ explain” individual storms and earthquakes. “Explanation” means re-describing the phenomena in the simplest manner possible, that is, in terms of space, time, gravitation, energy-transformations, chemical constitution of the materials concerned, etc. Such explanations can be (and indeed have been) made of the occurrences mentioned above, and the physical ideas we have noted are all that are included in the explanations.