When my love swears that she is made of truth,
I do believe her, though I know she lies,
That she might think me some untutored youth,
Unlearnèd in the world's false subtleties.
Thus vainly thinking that she thinks me young,
Although she knows my days are past the best, . . .
- "Vainly" -- what a wonderful pun! It is my vanity that drives me to this kind of deception, my sensitivity to the perception that I am past my first youth and no longer an attractively naive young lad. But even as I'm doing it, I can see that it is all in vain, because she knows the truth about me.
- So now he is imputing to her -- hopefully? -- the same kind of double-think existing in himself that he conveyed to us in the opening two lines. And at the same time as he hope she'll deceive herself into thinking him young, he knows his hope is in vain.
- Notice the word play on the verbs "think" and "know": the whole poem might be seen as an exploration of what these words mean. You could call it an epistemological love poem. (He thinks she thinks X although he knows she knows not-X.)