How do lines 39-41 in Act 2, Scene 3 of Shakespeare's Twelfth Night refer to the three elements of passion, thought, and emotion?
Feste the fool's song in Act 2, Scene 3 beginning at line 36 and ending at 42, encompassing lines 39-41 also, certainly does reveal a lot pertaining to the themes of passion, emotions, and thoughts. For one thing, the song is all about a woman searching for her true love and ceasing the journey once she finds him. The verse, "Your true love's coming," particularly refers to the passion involved in love because passion is a typical emotion in this type of relationship.
The verse in line 39, "That can sing both high and low," refers to other emotions beyond passion. The words high and low are often used to refer to emotional states. When one is said to be high, one's spirit's are high; or in other words, one feels happy. When one is said to be low, one feels gloomy, or sad or depressed. Hence, Feste is insightfully saying that the woman's true love will be a real person with real flesh and blood emotions, and Feste is using the words "high and low" to represent those human emotions. The true love will have both high and low emotions, or happy and sad emotions.
The theme of thought is really referred to in lines 41 through 42. Feste's lines, "Journeys end in lovers meeting, / Every wise man's son doth know," are sung with an ironic meaning. Throughout the play, Shakespeare equates the feeling of love with madness, showing us that love and thought, or wisdom, are two antithetical concepts. We especially see love being likened to madness when Olivia rejects Duke Olivia's love, even though she knows him to be a good and noble person, and falls in love with a man that really looks nothing like a man at all. Hence, while it's true that love's journey ends when two lovers meet, love and wisdom have nothing to do with each other, and rational thought also ends when love's journey ends. This is why Feste sings, "Every wise man's son doth know"; it's not the wise man who would fall in love, but rather his foolish son.