Shakespeare's fools are used for comic relief, but it also turns out that the fools are actually far more enlightened than the other characters of the play. The fool in Much Ado About Nothing is Dogberry. Dogberry is enlightened despite his stupidity because he is the only character in the play who has enough sense to capture and bring Don John and his comrades to justice, which brings the play to its resolution. Dogberry is also the most enlightening character because he reveals Shakespeare's theme of appearances vs. reality by showing us that none of the city's leaders actually are what they appear to be, especially himself.
Dogberry is an excessively vain and proud character. He is very proud of his role as Constable even though he does it very poorly. We see him perform his job very poorly when we first meet Dogberry in the third act, and he encourages his watch to "comprehend all vagrom," meaning to apprehend all vagrants, and to let go any man who will not stand in the prince's name (III.iii.22-25). He also encourages his watch to sleep rather than to talk as he "cannot see how sleeping should offend" (34, 37). Finally, he also tells his men to let thieves "steal out of [their] company" in order to tell that they are thieves (54-55).
While Dogberry seems to stand alone as a character, in actuality all of Messina's leaders possess the same vain excessive pride and care more about appearances than what is actually real. Governor Leonato is the one who was stupid enough to put Dogberry in his position as Constable, showing us just how foolish Leonato actually can be. Also, prince Don Pedro is still allowing his brother Don John to remain in his company even though Don John tried to overthrow his throne, which shows us just how foolish Don Pedro can be. Both of these decisions seem to have been made with the object of making their society seem, or appear, to be peaceful and strong, when in reality, their society is actually full of corruption. Since Dogberry resembles the other characters in foolishness and excessive pride, showing us the theme of appearance vs. reality, it makes Dogberry the most enlightening character as well as the most enlightened.
Despite the fact that Dogberry does his job poorly, he is actually the only one who discovers Don John's plot and brings Don John and his men to justice. Due to Dogberry, we witness Borachio confess to Claudio his fraudulent crime, saying:
I have deceived even your very eyes.
What your wisdoms could not discover, these shallow fools have brought to light, who in the night overheard me confessing to this man, how Don John your brother incensed me to slander the Lady Hero. (V.i.221-226)
Since Dogberry is the only character that rights wrongs in the play, it makes Dogberry the most enlightened character, as Borachio points out. The irony is that neither Dogberry nor his men actually meant to apprehend any criminals that night. The irony of the situation shows us that Dogberry is not only a fool, but a very enlightened fool.