The most important theme in A Midsummer Night's Dream is reality vs. illusion. All characters found in the play, the lovers, the fairies, and the mechanicals, present the theme of reality vs. illusion, showing us just how much reality is actually influenced by our illusions.
The theme of reality vs. illusion is best seen in how the fairies manipulate reality using an enchanted flower that has been hit by Cupid's arrow. Oberon changes reality by manipulating his wife into falling in love with a donkey in order to acquire the changeling Indian boy he is jealous of. Puck and Oberon also manipulate reality by creating a dreamlike state for the lovers, which at times is rather nightmarish. Puck creates a nightmare for Hermia by manipulating Lysander into falling in love with Helena; Puck also creates a nightmarish state for Helena by manipulating both men into falling in love with her because now she feels she is the object of a horrible joke and that even her best friend Hermia is in on it, as we see in her lines:
Lo, she is one of this confederacy!
Now I perceive they have conjoin'd all three
To fashion this false sport, in spite of me. (III.ii.195-197)
Puck further manipulates reality, creating a dreamlike state for the lovers, by finally repairing the damage he had created and pairing the couples correctly.
The mechanicals also let their illusions manipulate their reality. The mechanicals had illusions of presenting a well-acted play before Duke Theseus in honor of his wedding day. However, the reality is that they are actually too uneducated and too unskilled to be able to give a convincing performance. We especially see Bottom having visions of giving a grand performance and moving the audience to tears, as we see when he says that if he performs the role of Pyramus, "let the audience look to their eyes; I will move storms" (I.ii.21-22). Despite the mechanicals' illusions of their abilities, the reality is that they performed the play very poorly and ultimately turned a tragedy into something very comical. However, regardless of reality, they let their illusions govern their actions, thereby manipulating their own reality by allowing themselves to be ridiculed for a poor performance.