What Freud meant by “quantities” was the level of psychic energy flowing through the neurons. The energy arose from arousal either by a sensory organ or—far more important—by the body’s own drives. Discharging this energy—in the sex act, for instance—creates pleasure for the individual, whereas blocking its discharge creates displeasure.
(Scientific American Mind; Volume 17, Number 2; Neurotic about Neurons)
Can I say 'of' instead of 'by' here?