The authors draw a distinction between two kinds of defeasible conditional obligation: those in which the condition merely provides a reasonable indicator of the existence of the obligation (which they term ``epistemic''), and those in which the condition provides a ``reason for the existence of the obligation'' (called ``constitutive''). They see this as running roughly parallel to the familiar if controversial philosophical distinction, on the factual level, between statements of tendency and statements of causality. The authors construct a possible world semantics with selection functions, intended to reflect both of these kinds of defeasible conditional obligation.