Uncertainty

Quis scit, an adjiciant hodiernæ crastina summæ Tempora di superi?

 Who knows whether the gods will add to-morrow to the present hour?
       Horace—Carmina. IV. 7. 17.   1

Omnia sunt hominum tenui pendentia filo: Et subito casu, quæ valuere, ruunt.

 All human things hang on a slender thread: the strongest fall with a sudden crash.
       Ovid—Epistolæ Ex Ponto. IV. 3. 35.   2

Nothing is but what is not.

       Macbeth. Act I. Sc. 3. L. 141.   3 
               This

I ever held worse than all certitude, To know not what the worst ahead might be.

       Swinburne—Marino Faliero. Act V.   4 
 Dum in dubio est animus, paulo momento huc illuc impellitur.
 When the mind is in a state of uncertainty the smallest impulse directs it to either side.
       Terrence—Andria. I. 5. 32.