So as this only point among the rest remaineth sure and certain, namely, that nothing is certaine.

Pliny the Elder (AD 23-79) Roman author, naturalist, philosopher, military commander [Gaius Plinius Secundus]

Historia Naturalis [Natural History], Book 2, ch. 7 (AD 77-79) [tr. Holland (1601)]
    (Source)

Often paraphrased, "The only certainty is that nothing is certain."