Pliny's "Natural History" was written alongside other substantial works (which have since been lost). Pliny (AD 23–79) combined his scholarly activities with a busy career as an imperial administrator for the emperor Vespasian. Much of his writing was done at night; daytime hours were spent working for the emperor, as he explains in the dedicatory preface addressed to Vespasian's elder son, the future emperor Titus, with whom he had served in the army (and to whom the work is dedicated). As for the nocturnal hours spent writing, these were seen not as a loss of sleep but as an addition to life, for as he states in the preface, "Vita vigilia est", "to be alive is to be watchful", in a military metaphor of a sentry keeping watch in the night. Pliny claims to be the only Roman ever to have undertaken such a work, in his prayer for the blessing of the universal mother: