He thinks too much: such men are dangerous.

ANTONY.
Fear him not, Caesar; he's not dangerous;
He is a noble Roman and well given.

CAESAR.
Would he were fatter! But I fear him not:
Yet, if my name were liable to fear,
I do not know the man I should avoid
So soon as that spare Cassius. He reads much;
He is a great observer, and he looks
Quite through the deeds of men: he loves no plays,
As thou dost, Antony; he hears no music:
Seldom he smiles; and smiles in such a sort
As if he mock'd himself and scorn'd his spirit
That could be moved to smile at any thing.
Such men as he be never at heart's ease
Whiles they behold a greater than themselves;
And therefore are they very dangerous.
I rather tell thee what is to be fear'd
Than what I fear, for always I am Caesar.
Come on my right hand, for this ear is deaf,
And tell me truly what thou think'st of him.

[Exeunt Caesar and his Train. Casca stays.]

CASCA.
You pull'd me by the cloak; would you speak with me?