A brief outline of nineteenth-century neurophysiology is presented, indicating that the nerve action potential was already known, through the work of Du Bois Reymond, von Helmholtz and Bernstein, by the time Freud wrote the 'Project'. Their research suggested that action potential could propagate as a wave of depolarization without physical transfer or accumulation of energy. By that time central inhibition was also known to be a process based on experimental findings.

The paper discusses evidence indicating that the quantity of energy or Qe which operates in the nervous system is intended by Freud in the 'Project', to be a mechanical metaphor not taking into account the neurophysiological knowledge of his time. Moreover, as Qe always produced facilitation, inhibition as such is not recognized by Freud and is represented only as a deviation or side-cathexis of facilitation. This concept of inhibition obviously has nothing to do with neurophysiology.