Full Idea
Freud argued that the passions in general …were the pressures of a yet unknown 'quantity' (which he simply designated 'Q'). He first thought this flowed through neurones, …and always couched the idea in the language of hydraulics.
Gist of Idea
Freud said passions are pressures of some flowing hydraulic quantity
Source
report of Sigmund Freud (works [1900]) by Robert C. Solomon - The Passions 3.4
Book Reference
Solomon,Robert C.: 'The Passions (1993 ed)' [Hackett 1993], p.81
A Reaction
This is the main target of Solomon's criticism, because its imagery has become so widespread. It leads to talk of suppressing emotions, or sublimating them. However, it is not too different from Nietzsche's 'drives' or 'will to power'.