Lady, cucullus non facit monachum; that’s as much
to say as I wear not motley in my brain. (I.v.)

Here, Feste defends himself after Lady Olivia calls him a “dry fool” and insists that he be taken away. In Latin, Feste tells Olivia that “the cowl does not make a monk,” essentially implying that she shouldn’t judge a book by its cover. Just because Feste looks the part of a fool does not mean that he is actually foolish or unintelligent. As Feste insists, “he wears not motley in his brain,” meaning that he still has his wits about him. As we quickly learn, Feste is the most intelligent and incisive character in Twelfth Night. His brand of foolishness consists of funny puns and innuendos, but these are often quite poignant. By masking his wisdom with “foolishness,” Feste’s biting commentary feels unassuming and can squeak by without eliciting offense from those in power.

…This is a practice
As full of labour as a wise man’s art,
For folly, that he wisely shows is fit,
But wise men, folly-fall’n, quite taint their wit (III.i.)

Here, Viola remarks that Feste’s witticism requires a great deal of intelligence and skill, and is comparable to the work of a scholar or wise man. Viola is one of the few characters in the play who appreciate or understand the skill and wisdom that Feste possesses. She suggests that cleverly-placed foolishness reinforces one’s claim to wisdom, whereas wise and prudent men who act foolishly do permanent damage to their reputation. The line can also be read as a reference to Malvolio, who throughout the first acts of the play adopts the affectation of a sober and highly-educated scholar, but does irreparable damage to himself once he exposes the vulgar scope of his ambitions. The line suggests there is more freedom in being overlooked by the world as a “fool,” than being regarded as a wise man. And sure enough, among all the characters in Twelfth Night, Feste enjoys the full freedom to speak his mind.

But as well? Then you are mad, indeed, if you be no
better in your wits than a fool (IV.ii.)

Here, Feste is speaking to Malvolio, who has been locked away in prison for his erratic behavior, which has been construed as madness. Malvolio insists that he is in fact sane and that he possesses as much wit as the next man, including Feste. Hearing Malvolio, Feste pounces on the opportunity to strike back and avenge Malvolio’s previous insults. Until this point, Malvolio has been openly dismissive of Feste because he plays the role of fool or jester, and even goes so far as to suggest that wise men who delight in the company of fools are no better than the fools’ “zanies,” or students. Feste can now rub it in, contending that if Malvolio has as much wit as a “fool” like him, then he really must be mad. In a sense, Feste is now using Malvolio’s logic against him. Again, this line speaks to the various interpretations of what it means to be a fool in Twelfth Night, suggesting that the distinction between wisdom, foolishness, and madness is quite fluid.