She was aware only of the strength, and not of the medium, and when she seemed most carried away by what he had written, in reality she had been carried away by something quite foreign to it—by a thought, terrible and perilous, that had formed itself unsummoned in her brain.
In this moment, I believe we can compare Ruth's feelings towards Martin with her feelings towards his work. From the beginning of their affair, Ruth had a strange obsession with Martin's strength and muscular neck. Upon meeting Martin "Her gaze rested for a moment on the muscular neck, heavy corded, almost bull-like, bronzed by the sun, spilling over with rugged health and strength... It seemed to her that if she could lay her two hands upon that neck that all its strength and vigor would flow out to her." (Pg. 10) While she is aware that she does not desire him as a life partner, she seems to be "carried away" by her fascination with his physical state. In the end when Ruth approaches Martin with desire for his intellect "she had been carried away by something quite foreign to it"; this, I assume, would be Ruth questioning her distraction with Martin's neck and misinterpreting it as a desire for his love. This ongoing misconception leads Martin to believe that he really will be with Ruth one day. As she recognizes here, her feelings towards Martin seem "unsummoned in her brain."