[2]The action to which the "ought" applies must indeed be possible under natural conditions.
[4] There are several ways of deriving the formula—for example, the argument that it is wrong to blame people for things that they cannot control (essentially phrasing the formula as the contrapositive "'cannot' implies 'has no duty to'").
[5] This ethical formula can be expressed in deontic logic with the multimodal axiom:
However, in practical situations, obligations are usually assigned in anticipation of future events, in which case alethic possibilities can be hard to judge; Therefore, obligation assignments may be performed under the assumption of different conditions on different branches of timelines in the future, and past obligation assignments may be updated due to unforeseen developments that happened along the timeline.
[citation needed] "Ought implies can" is logically equivalent to the formula