I conceive what happens when we see an object more or less on the following lines. For the sake of simplicity, let us take a small self-luminous object. In this object, a certain number of atoms are losing energy and radiating it according to the quantum principle. The resulting light-waves become superposed according to the usual mathematical principles; each part of each light-wave consists of events in a certain region of space-time. On coming in contact with the human body, the energy in the light-wave takes new forms, but there is still causal continuity. At last it reaches the brain, and there one of its constituent events is what we call a visual sensation. This visual sensation is popularly called seeing the object from which the light-waves started--or from which they were reflected if the object was not self-luminous. Thus what is called a perception is only connected with its object through the laws of physics. Its relation to the object is causal and mathematical; we cannot say whether or not it resembles the object in any intrinsic respect, except that both it and the object are brief events in space-time. I think we may lay down the following universal characteristics of causal laws in an advanced science. Given any event, there are other events at neighbouring places in space-time which will occur slightly later if no other factors intervene; but in practice other factors almost always do intervene, and, in that case, the event which actually occurs at any point of space-time is a mathematical resultant of those which would have followed the various neighbouring events if they had been alone concerned. The equations of physics give the rules according to which events are connected, but all are of the above sort. Formerly it was thought that the equations of physics sufficed, theoretically, to determine the course of affairs in the physical world, given all the facts about some finite stretch of time, however short. Now it appears that this is not the case, so far as the known equations are concerned. The known equations suffice to determine what happens in empty space, and statistical averages as to what happens to matter; but they do not tell us when an individual atom will absorb or radiate energy. Whether there are laws, other than those of statistics, governing the behaviour of an individual atom in this respect, we do not know. It should be observed that there are causal laws of a different sort from those of pure physics; such are the laws that light-waves “cause” visual sensations and sound-waves “cause” auditory sensations. All the empirical evidence for physics rests upon such laws, therefore nothing in physics can have a higher degree of certainty than such laws have. Let us stop a moment to ask what we mean by “cause” in this connection.