When, my love swears that she is made of truth
The nature of truth and flattery in romantic relationships is the theme of sonnet 138, often thought of as a song.
All of Shakespeare's Sonnets have titles which are basically the first line of 14 (in most cases anyhow, as some of the poems break the rules and have 15 lines!)
No one knows exactly when the sonnets were written, however, 138 was published in The Passionate Pilgrim, by William Jaggard in 1599, which was like one of today's anthologies.
Ten years later, they were all published by Thomas Thorpe, though laden with errors, it is unlikely Shakespeare knew of it!
137 is about contradictions. The poet's heart believes the dark lady is his alone but he knows that it is not true and he is guided by blind love. At the same time, he implies that she is a common prostitute.
In 139, the speaker elaborates the theme of untruth. Unfaithful she may be, the speaker rationalizes by insisting she gives him his full attention when they are together and ignoring what may well happen when they are apart.
The speaker in Sonnet 138 confesses to a less than perfect relationship based on lies and deceit of which each partner is aware, yet they continue to flatter each other. They remain together for sex and because they are both comfortable with the lies.
In the first quatrain, the speaker pretends to believe his mistress, knowing full well she is lying. At the same time, he is pretending to her, that he is younger. For example, "she is made of truth, I do believe her."
"Made" could mean maid or virgin, which is what he wants to believe but he "knows she lies" which may also mean she "lays" with other men - sleeping around!
At the same time, he refers to himself as "untutored youth" then "unlearned" reinforcing this point of being a young man inexperienced in love.
In quatrain 2, the poet develops his argument. "Vainly" means she falsely "thinks me young." She knows he is "past the best" and he knows of "her false-speaking tongue."
"Simple I credit" is pretending to believe her and "simple truth supprest," paradoxically, is that they are both refusing to acknowledge the self-evident truth.
In the last quatrain, both are lying and cannot trust each other. For example, she is "unjust" compared to him being "old."
The couplet spins the poem around with some wry humor. They "lie" with each other and lay together in bed, reminiscent of quatrain one. The couple in the couplet are together and are flattered by each other's funny little ways.
All is forgotten in the couplet with a good old bit of sex at the end to resolve any problem, as the relationship is purely physical anyway. Who cares about the deceit?
This sonnet is part of the dark lady set in the series (127 onwards). Towards the end of the entire 154 long sequence, the poet has pretended his mistress is pure and innocent and pretended he is young and virile, in their little tit for tat game.
Age and deteriorating beauty has thrown some frustration into Shakespeare's thinking at this stage of his life, and this is reflected by his Sonnet 138!