 where the \(F\text{s}\) cover the \(G\text{s}\) if and only if every part of every \(G\) has a part in common with an \(F\). Similarly, suppose a train is a maximal string of railroad carriages, all of which are connected to one another. We may define a functional expression, ‘the train of \(x\)’, by means of an ‘abstraction’ principle: The train of \(x\) = the train of \(y\) if and only if \(x\) and \(y\) are connected carriages. We may then say that \(x\) is a train if and only if for some carriage \(y\), \(x\) is the train of \(y\). The simple account thus yields the consequence that trains are to be reckoned abstract entities.