 For the simple past, both event time and reference time are situated on the time line before speech time. In case of the perfect, R and S are simultaneous and E is earlier than both R and S. Intuitively, reference time represents the perspective from which an eventuality is perceived on the time line. This is not a purely semantic theory of tense like temporal logic, rather it is a pragmatic theory of tense. In particular, reference time (in contrast to event time) has to be mutually known by communication partners. When using the present perfect, reference time is known since it coincides with speech time. However, reference time is not necessarily known when the past tense is used, since it is just required to be earlier than speech time and to coincide with event time. This may explain why sentences in the past sound strange when uttered out of the blue. Steedman (1997) presents the following examples: 