Someone said somewhere in something I read, "Shakespeare's characters do not have I.Q.s." I found this very interesting. The concept of measurable intelligence is fairly new. I believe it started around the time of World War I because the Army wanted to know where to assign recruits. In Shakespeare's time they seemed to think in terms of "reason." Some people had the ability to reason while others didn't. A character like Caliban in The Tempest is a savage, but he seems to be able to express himself eloquently. In fact, many characters of low status, such as the gravedigger in Hamlet and the soldier who describes how courageously Macbeth conducted himself in the recent battle, are able to express themselves in colorful language, even though they would probably score below 100 on an I.Q. test.
Brutus undoubtedly has a high I.Q., not unlike Hamlet. Which means, in answer to your question, that Brutus is intelligent. But he is not "street smart." He is not "savvy." He is not "cool." He is not "hip." He lacks "moxie." He is a philosopher at heart. Like Hamlet, he is forced into a position for which he is not suited. Hamlet says as much in one of his soliloquies. If he had his wishes, he would be a scholar at Wittenberg. He loves books, solitude, meditation. In Shakespeare Brutus resembles Hamlet, Prospero in The Tempest, and Richard III. He is an introvert--at least as he is portrayed by Shakespeare. He may be more intelligent than any of the other men in the play, but he lacks "cunning," which is described by the great German philosopher in the following passage:
It is just the lower grades of intellectual superiority, such as shrewdness, cunning, and definite but one-sided talents that enable one to get on in the world and readily establish one’s good fortune, especially when impudence and effrontery (like the audacity just mentioned) supplement such talents.
Cassius obviously underrates Brutus, although he praises him outlandishly. Cassius has no way of assessing Brutus's I.Q., which would be around 140 or 159. Cassius thinks he can use Brutus as a figurehead for his conspiracy to assassinate Julius Caesar, but he is cunning and not intelligent. He does not foresee that Brutus will want to institute philosophical and idealistic government policies in Rome after the conspirators have temporarily achieved power. Cassius thought it would be easy to manipulate Brutus, but he finds out almost immediately after the assassination that Brutus is guided by an idealistic vision, derived from all his reading, including Plato's Republic, which he insists on imposing, however impractical and unrealistic in the vicious world of the time.
The poet, literary and social critic, T.S.Eliot observed, "Between the idea and the reality....Falls the Shadow,"words that pose the problem associated with leaders whose principles are based upon ideology. For, realities can be entirely different from the idealist's perception of them.
That Brutus is idealistic is indicated in his soliloquy in his orchard in which he considers what he has seen of Caesar the day before in his procession through the streets of Rome when his personal desires ruled him rather than concern for the state. He concludes that Caesar may well become a tyrant:
And therefore think him as a serpent's egg
Which hatched would as his kind grow mischievous (2.1)
Because he considers Caesar as a potential threat to a partially democratic state in the republic which Brutus supports, he decides that Caesar's threat of tyranny is sufficient for an assassination.
The idealist state of Brutus's mind causes him to misjudge Marc Antony, as well. Were he "smart" as is Cassius, he would have Antony killed, too. But, because his heart is noble and idealistic, he tends to lose touch with the brutal realities of life--realities that Cassius understands clearly.
In another example of Cassius's being "smart"--as meaning that one is aware of the realities of situations--when Brutus is not is the argument that the two have about the forthcoming battle at Philippi. For, whereas Cassius speaks in realities, telling Brutus that the soldiers are fatigued and if they march to Phillipi, there will be too enervated to win the battle. But, Brutus idealistically speaks in metaphor.
Our legions are brimful, our cause is ripe....
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
And we must take the current when it serves,
Or lose our ventures. (4.3)
Of course, because Brutus refuses again to listen to Cassius, their troops are defeated, and they lose their lives. Therefore, in the sense that Brutus is too idealistic and not practical in his reasoning, he is not "smart," and the "Shadow" of death falls upon him as a result.