Editing Abstract and concrete (section)

===Abstract objects and causality===
Another popular proposal for drawing the abstract–concrete distinction contends that an object is abstract if it lacks [[causality|causal]] power. A causal power has the ability to affect something causally. Thus, the empty set is abstract because it cannot act on other objects. One problem with this view is that it is not clear exactly what it is to have causal power. For a more detailed exploration of the abstract–concrete distinction, see the relevant ''[[Stanford Encyclopedia of Philosophy]]'' article.<ref name=SEP>{{cite SEP |url-id=abstract-objects |title=Abstract Objects |last=Rosen |first=Gideon}}</ref>