There are two main themes in Shakespeare's Sonnet 138. The two main themes developed in the sonnet are lies and truths.
The theme of lies is developed through the double meaning used in the word "lies" within the text:
When my mistress swears that she is faithful/ I do believe her though I know she lies
Here, the speaker uses the word "lies" is a polysemy (has a double meaning). The word lies refers to the fact that she is not telling the truth and that she physically lies with other people.
Therefore, this idea of lying is thematically created through the repetitive use of the words which coincide with the term lie: deceit, foolishly, untruths, ugly truth, and disguise.
To mirror this theme, the theme of truths exists within the poem as well; although, this theme is more implicit. The speaker admits that he knows his mistress is lying, but his acceptance of her lying creates a truth for him. This theme is defined through the use of the following words: believe and pretense of truth.
It is through each's acceptance of lies and omittances of truths which both the speaker and the mistress find truth in their realtionship.
The poem deals mainly with dishonesty and loyalty. Throughout the sonnet, Shakespeare focuses the reader's attention on the dishonest relationship the speaker shares with his lover. Both lie to each other about different aspects of their acquaintance and the nature of their commitment.
It is as if the couple is playing games with one another; both are dishonest in what they say and what they claim they believe. In lines one and two, for example, the speaker confesses that, even though he knows that his young lover is lying when she tells him that she is the epitome of veracity, he acts as if he believes she is telling the truth. He does this so she believes he is an inexperienced youngster, even though both of them know he is not.
The speaker emphasizes this fact in the second line of the second quatrain, stating,
Simply I credit her false speaking tongue
The speaker then acknowledges the fact that, in this manner, they both suppress the real truth. He rhetorically asks why she does not admit to being biased and why he does not do the same by acceding that he is, in fact, old. The answer is obvious, as he says in the next two lines. Love is at its best if it seems to be based on a truth, and when in love, one does not wish to be reminded of one's seniority.
It seems, therefore, that one can love with greater passion if the love is based on a lie and that dotage, as in the speaker's case, is preferable than being made aware of the discrepancy in age between lovers. It is a paradoxical situation. Both parties, however, seem to be happy with the arrangement, thrive in one another's presence, and remain loyal to each other.
The speaker emphasizes this particularity in the rhyming couplet by stating that he and his lover keep company and sleep with each other and are happy in this situation although they are both at fault. What binds them is the fact that they are flattered by each other's dishonesty.
Therefore I lie with her and she with me,
And in our faults by lies we flatter'd be.
Shakespeare effectively conveys his message by using the word 'lie' as a pun throughout the poem to refer to the physical act of sleeping with another (a sexual connotation) and the telling of untruths. This play on the word "lie" makes the sonnet both humorous and thought-provoking and effectively denotes the paradox on which the speaker and his lover's relationship is built.
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