We at the height are ready to decline.
There is a tide in the affairs of men
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
On such a full sea are we now afloat,
And we must take the current when it serves,
Or lose our ventures. (IV.ii.269–276)
Throughout the play, the theme of fate versus free will proves important: here, Brutus suggests that both exist and that one should take advantage of fate by asserting one’s will. While subsequent events demonstrate that the force of fate (or perhaps just Antony and Octavius’s superior maneuvering) is stronger than Brutus’s individual actions, his speech still makes for a graceful, philosophic axiom, showing Brutus to be a man of deep reflection.
Brutus cannot sleep—perhaps because he is brooding internally on his guilt; in any case, this guilt is soon manifested externally in the form of the Ghost of Caesar. This phantom’s identification of himself to Brutus as “thy evil spirit” could mean either that the Ghost is an evil spirit appearing to Brutus’s eyes only—a spirit that is “his” alone—or that the Ghost represents Brutus’s own spirit, which is secretly evil (IV.ii.333). However one interprets the arrival of the specter, the event can only bode ill for Brutus in the battle to come.