When my love swears that she is made of truth,
I do believe her, though I know she lies
The speaker’s lover swears that she is completely truthful (with a pun on the idea that she is a “maid of truth,” meaning a true virgin). The fact that she has to “swear” this, however, casts doubt on how true it is. Similarly, the speaker knows it is a lie (his lover is not faithful to him). Yet he chooses to believe it anyway.
Thus vainly thinking that she thinks me young
The speaker chooses to believe his lover’s unconvincing lies about being faithful because it flatters his vanity. He is self-conscious about his age, so he chooses to be tricked by his lover’s lies the way only a young, inexperienced man who is “unlearnèd in the world’s false subtleties” could be. The thought that his lover thinks him to be young allows him to forget his actual age.
Simply I credit her false-speaking tongue
Instead of condemning his lover for lying, he “credits” her. This can mean that he appreciates (or gives her credit) for her lies, and also that he believes them. At the same time, because “credit” is a financial metaphor, it suggests that their relationship is based on an exchange: he believes her lies because she believes his lies. The idea of a “false-speaking tongue” is an allusion to the serpent in Genesis that tempts Eve to eat from the Tree of Knowledge. This gives a darker meaning to the line, suggesting that the result of all this lying could be a disastrous one.
Therefore I lie with her and she with me,
And in our faults by lies we flattered be.
The lovers both "lie" (tell untruths) to each other and "lie" (have sex) with each other. The sonnet’s final couplet suggests that there is an important relationship between telling untruths and being in love. It is the untruths that make the romance possible. Lies convert all of the lovers’ “faults” (infidelity or old age) into things they can feel “flattered” about. Lies are the foundation of their love.
