How logical is the nymph's reply?

The nymph’s refutation may be schematized in a logical
pattern: 1. If a, then b. 2. Not a. 3. Therefore, not b. The 1 is “if the world were
young, etc., then I would live with you” (brought out in stanza 1). The 2 is all the
negative detail in stanzas 2–5. The 3 is the final stanza, in which she draws the
conclusion of the previous stanzas. The world is not eternally young and pleasant, and
therefore she rejects the shepherd; in fact, however, even if the world were perfect,
she does not concede to the shepherd that he would be her choice (“my mind might move,”
line 24). This a very logical argument that can be understood in an elementary
rhetorical class or on paper if one were to draw a
simple flow chart.