Eve "falls" in this chapter, bringing humankind as a whole out of joint with God. Even before the fall, she is depicted by Milton as a woman with weaknesses. She is vain about her beauty, for she likes to gaze at her reflection in water, and she is portrayed as using her feminine wiles to beguile Adam into allowing them to be apart in the Garden against his better judgment. Even before the fall, there is something wanton and slightly sexualized and manipulative about Eve. She also tends to overestimate herself, even while treating Adam as the wiser of the two.
The serpent (Satan) hones in on these weaknesses and goes straight to work as soon as he finds Eve alone. He flatters her beauty and charm, as well as her ability to reason. While his reasoning for why she should eat the fruit is faulty, she is convinced by it, and does the fateful deed.
Eve is then shown taking Adam down with her, using his love of her to persuade him to share her fate.
Although Adam is presented in Paradise Lost as having a greater capacity for reason than Eve, she makes a logical suggestion in Book IX that if she and Adam separated for a time, they would not be distracted by each other and would be able to get more work done. Adam admires her suggestion but says that God did not create them for hard labor:
For not to irksom toil, but to delight
He made us, and delight to Reason joyn'd. (242-43)
But Adam supposes that a short absence is alright, reasoning that "a short retirement urges sweet return" (distance makes the heart grow fonder). Adam does warn Eve of the envious Satan. Eve replies that she can be trusted. Adam agrees, saying he trusts her; it is Satan ("our Foe") whom he does not trust, especially considering the fact that Satan was able to seduce angels. Eve replies that to be happy in Eden, they should not live in fear.
Even though Adam insists that they are better off together than apart, Eve argues that Satan will tempt Adam first. Adam having more reason, will be a greater challenge for Satan; Satan's pride will make him want to accept this challenge if it ever comes to that.
But Satan does tempt Eve first and Eve's ability to reason is no match for Satan's/Serpent's ability to manipulate reason. In love, Eve decides to share the fruit with Adam. In love, he decides to be lifted or doomed with her. And in a sense, with her love and "Femal charm," she tempts Adam. Thus, both Adam and Eve are guilty of being deceived into sinning. She is deceived by false reason; he is deceived by her love and charm.
With liberal hand: he scrupl'd not to eat
Against his better knowledge, not deceav'd,
But fondly overcome with Femal charm. (997-99)
Although Eve is the first to sin, she makes a reasonable argument that Adam is as guilty. First she says that he would have been deceived just as she was; the Serpent was just that clever. Then she says that if he (Adam) truly knew Eve's weaknesses and the power of the "Foe," he would never have let her go off alone. Book IX ends with the two blaming each other, both fallen heroes. Unlike traditional criticism which had put the bulk of the blame of Original Sin on Eve's shoulders, Eve makes a convincing argument that Adam is equally guilty.