That philosophical themes could be studied in an exact manner by logical
meanS was a delightful discovery to make. Until then, the only outlet
for a philosophical interest known to me was the production of poetry or
essays. These means of expression remain inconclusive, however, with a
tendency towards profuseness. The logical discipline provides so me
intellectual backbone, without excluding the literary modes. A master's
thesis by Erik Krabbe introduced me to the subject of tense logic. The
doctoral dissertation of Paul N eedham awaked me (as so many others)
from my dogmatic slumbers concerning the latter's mono- poly on the
logical study of Time. Finally, a set of lecture notes by Frank Veltman
showed me how classical model theory is just as relevant to that study
as more exotic intensional techniques. Of the authors whose work
inspired me most, I would mention Arthur Prior, for his irresistible
blend of logic and philosophy, Krister Segerberg, for his technical
opening up of a systematic theory, and Hans Kamp, for his mastery of all
these things at once. Many colleagues have made helpful comments on the
two previous versions of this text. I would like to thank especially my
students Ed Brinksma, Jan van Eyck and Wilfried Meyer-Viol for their
logical and cultural criticism. The drawings were contributed by the
versatile Bauke Mulder. Finally, Professor H intikka's kind appreciation
provided the stimulus to write this book.