The fault (dear Brutus) is not in our stars,
But in ourselves that we are underlings (I.ii)
Here, again, trying to enlist Brutus in the murder plot, Cassius suggests that it is not their fate to be Caesar’s underlings, but rather that it is within their power to alter the course of history. This quote is important as it introduces one of the central conundrums of the play: how much of the future can an individual influence on his/her own and how much is destined to happen anyway? Despite the attempts of Cassius and Brutus to restore the republic and abolish tyranny, the result is ultimately the same; they are destined to fail.
Go bid the priests do present sacrifice
And bring me options of success (II.i)
Here, Caesar orders his servant to order a sacrifice from the priests, as a way to determine whether or not he should go to the Senate. By asking for sacrifice to be performed, Caesar reveals his belief in predestination. The entrails of the ox will offer signs of what is fated to happen, and what he ought to do in accordance. Even though the ox appears to have no heart (an ominous portent) Caesar misreads it as an indictment of his cowardice should he stay home. Fate and predestination both have a large presence in the play, often giving characters subtle signs and glimpses of what is to pass. Yet the characters fail, rather consistently, to interpret these symbols correctly.
There is a tide in the affairs of men
Which, taken at the flood, leads on to fortune.
Omitted, all the voyages of their life
Is bound in shallows and miseries (IV.iii)
Here, Brutus explains to Cassius that it is advantageous to march forth and surprise the Octavius-Antony faction at Philippi. Through the metaphor of a flood, the quote speaks to a delicate balance between fate and free-will, and (with Machiavellian echoes) implies there are moments when destiny offers rare opportunities that should be seized. Essentially, Brutus is implying that free-will must be exercised judiciously, when it properly compliments the course of destiny. Again, Brutus here misreads fate’s trajectory, and by exerting his will, ensures his own demise.