) is followed by thunder (
). But the separate flashes are particulars, not identical, but sharing the common property of being lightning. The only way of expressing a common property generally is to say that a common property of a number of objects is a propositional function which becomes true when any one of these objects is taken as the value of the variable. In this case all the objects are "instances" of the truth of the propositional function—for a propositional function, though it cannot itself be true or false, is true in certain instances and false in certain others, unless it is "always true" or "always false." When, to return to our example, we say that
is in every instance followed by
, we mean that, whatever
may be, if