Very often it is proved experimentally that two electrified bodies repel or attract each other, and from this fact is deduced the conclusion that there exist two kinds of electricity of such properties that bodies of the same kind of electricity repel and bodies of different kinds attract each other. This proceeding is not quite correct.

Example. If we consider two integral numbers, then their difference is divisible by 3 or not. But it is obviously wrong to deduce from this fact that we can divide the set of all integral numbers into two such classes, that the difference of two numbers belonging to one class is divisible through 3 and the difference of two numbers belonging to different classes is not. It is known that there exist three classes possessing these properties.