``Hans Kamp devised in 1967 a consistent semantic interpretation for `now' which can be presented, with slight modifications, as a new sort of UT-calculus, in which *T *ties each tense-logical proposition not to one instant but to two, i.e. our basic form is not *Tap* but *Tabp.* The proposition *p* is related to the instants *a* and *b* in different ways; the essential difference is that the elimination of complexities from what is put after *Tab* may take us to other instants than *a*, but never to other instants than *b*. And wherever we may have been taken from *a* by operators like *G* and *H*, the one place to which we are always immediately taken by *J* [`It is now the case that'] is the instant *b,* i.e. the instant represented by the second argument of *T*. We might read the form *Tabp* as `From *b* it is the case at *a* that *p*', and `From *b* it is that case at *a* that *p--*now' = `From *b* it is the case at *b* that *p*'." -- Prior, A. N.: 1968, `"Now"', Nous 2(2), 101–119.

The 1967 work that Prior refers to is Kamp, H.: 1967, "The treatment of `now' as a 1-place sentential operator", multilith circulated to a graduate seminar at the University of California in Los Angeles. The results were reported in A.N. Prior (1968) and full treatment was published in Kamp (1971),`The formal properties of ``Now"', *Theoria* 37, pp. 227-274.

Prior presents problematic sentences such as,

(A) It will be the case that it is now the case that I am sitting,

and attributes to Kamp the above ``the first solution offered" to such problems. Prior agrees that this works but goes on to present an alternative (singly-indexed) treatment by introducing an instant-constant *n* for a designated instant (and then doing some other tricks due to Prior & Meredith (1953) -- I don't think he just gives extensional treatment of the operators but it seems very similar).

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