 An arrow from Y to Z in a DAG represents that Y is a direct cause of \(Z.\) Roughly, this means that the value of Y makes some causal difference for the value of Z, and that Y influences Z through some process that is not mediated by any other variable in \(\bV\). Directness is relative to a variable set: Y may be a direct cause of Z relative to variable set \(\bV\), but not relative to variable set \(\bV'\) that includes some additional variable(s) that mediate the influence of Y on \(Z.\) As we develop our account of graphical causal models in more detail, we will be able to say more precisely what it means for one variable to be a direct cause of another. While we will not define “cause”, causal models presuppose a broadly difference-making notion of causation, rather than a causal process notion (Salmon 1984, Dowe 2000) or a mechanistic notion (Machamer, Darden, &amp; Craver 2000; Glennan 2017). We will call the system of direct causal relations represented in a DAG such as  Figure 1  the causal structure on the variable set \(\bV\).