Deception is the most prominent idea in Sonnet 137. The speaker in the poem thinks everybody has deceived him. First, he complains that Love has deceived him, then he moves on to his own heart and eyes. And, of course, throughout the whole sonnet he’s indirectly complaining about his deceitful mistress. In every case, unfaithfulness is at the heart of his gripe.
Line 1: The first person the speaker blames for deceiving him is the god of "Love." The words the speaker addresses to Love are a clear instance of apostrophe, the rhetorical technique in which the speaker of a poem addresses some absent person or entity. You could also say that calling Love "thou blind fool," as well as being an insult, is an allusion to traditional representations of the god Cupid as wearing a blindfold (as you can see in this Renaissance image).
Line 2: The speaker continues to vent his frustration at Love in the second line by pointing out a paradox. Now, thanks to Love’s meddling, his eyes "behold and see not what they see." At least part of this paradox can be explained by saying that the speaker is making a pun, a form of wordplay where a word means two different things. (The first time the word "see" is used in this line, it means "understand." The second time it is used, at the end of the line, it has its normal meaning. Thus, the phrase "see not what they see" actually means "do not understanding what they are looking at.") Oh yeah, and one more thing. Do you think Shakespeare’s speaker really expects Love to give him an answer to the question he is asking here? We don’t think so. Thus, lines 1-2 together count as a rhetorical question.
Line 4: You can tell how emotional the speaker is getting by the types of rhetorical devices he uses. To show just how angry he is, in this line he busts out the tried-and-true device known as hyperbole, exaggerating for effect. Where’s the exaggeration here? The speaker tells us that his eyes mistakenly believe that the worst thing in the world is the best thing in the world, or, as he phrases it in the poem (a bit more confusingly), "what the best is [my eyes] take the worst to be." Really? Do you really mean that? I mean, sure your girlfriend cheated on you, but there are some pretty nasty people out there—you know, tyrants and murderers and the like. Surely they are worse than your girlfriend. Thus, we would say that this part of line 4 counts as hyperbole. The fact that his eyes think that such a person is the "best" just goes to show the wild mood-swings of emotion that lovers can experience.
Lines 5-7: In the second quatrain, the surface message continues to blame Love for getting the speaker hooked on his untrustworthy mistress. You can see this in the use of another apostrophe when the speaker asks why "thou" (Love) "hast forgèd hooks" out of "eyes' falsehood" that keep his heart attached to his lady. At the same time, though, the speaker also kind of foreshadows the turn he will make in line 9 to blaming his own heart (and, in line 11, blaming his eyes) for the deception. This foreshadowing comes in the reference to how his eyes have become "corrupt by over-partial looks." The word "over-partial" in this context is, in fact, a pun—the use of one word with multiple meanings. As we talk about in more detail in our "Detailed Summary," this word can be interpreted in multiple ways. The most obvious takes "partial" as the opposite of "impartial," so that Shakespeare’s speaker is saying that either his own attachment to his lady or visible attachment to him has corrupted his "looks."
That said, we also think there’s a strong case to be made for the second, more hidden meaning. This meaning centers around the idea of "partial" as referring to a body-part. In this case, we think it’s more likely that Shakespeare’s speaker is talking about himself rather than his lady. According to this interpretation, what Shakespeare is saying here is that his eyes have been made corrupt by looks that are overly influenced by a certain part of anatomy (i.e., that he is "thinking with his you-know-what"). In this way, the speaker starts to put the blame for the deception on himself—or part of himself, anyway.
Lines 9-12: In these lines, the speaker finally stops cursing the god of Love for his troubles, and instead points the finger of blame at his own eyes and heart. The main rhetorical device used in these lines is the rhetorical question, a question you ask even though you don’t expect to get an answer. There are two rhetorical questions in these lines, one running from lines 9-10, in which he asks why his heart keeps thinking that his lady-friend is loyal to him even though it knows she isn’t. The second question runs from lines 11-12 and asks why his eyes keep telling him that his lady-friend is loyal to him, even when they know that his heart is being deceived. The speaker amps up his anger at this point by some careful use of alliteration, first in line 10, on "Which," "wide" and "world," then in line 11, on "seeing" and "say," and then, most forcefully of all, in line 12, where his alliteration on the letter "f" highlights the contrast between the definitions of "fair" and "foul."