terms as the argument to our propositional function. For example, "if Socrates is a man, Socrates is mortal," is necessary if Socrates is chosen as argument, but not if man or mortal is chosen. Again, "if Socrates is a man, Plato is mortal," will be necessary if either Socrates or man is chosen as argument, but not if Plato or mortal is chosen. However, this difficulty can be overcome by specifying the constituent which is to be regarded as argument, and we thus arrive at the following definition:
"A proposition is necessary with respect to a given constituent if it remains true when that constituent is altered in any way compatible with the proposition remaining significant."
We may now apply this definition to the definition of causality quoted above. It is obvious that the argument must be the time at which the earlier event occurs. Thus an instance of causality will be such as: "If the event e1 occurs at the time t1 it will be followed by the event e2." This proposition is intended to be necessary with respect to t1, i.e. to remain true however t1 may be varied. Causality, as a universal law, will then be the following: "Given any event e1 there is an event e2 such that, whenever e1 occurs, e2 occurs later." But before this can be considered precise, we must specify how much later e2 is to occur. Thus the principle becomes:—
"Given any event e1, there is an event e2 and a time-interval τ such that, whenever e1 occurs, e2 follows after an interval τ."
I am not concerned as yet to consider whether this law is true or false. For the present, I am merely concerned to discover what the law of causality is supposed to be. I pass, therefore, to the other definitions quoted above.
