 The two contributions are variously described. The male contributes a source of movement or dunamis (‘power’ or ‘potential’), which, as the argument unfolds, turns out to be a special sort of capacity to heat present in the semen’s pneuma or air, which is part of its nature. (This gets fully worked out in GA II 2–3) The semen itself is mere a vehicle for delivering this heating capacity; the male makes no material contribution to the offspring. The female contributes what might be called ‘prepared matter’; all it needs is the presence within it of the heating capacity from the male and it begins a more or less lengthy and complicated developmental process, which Aristotle analogizes to a sort of automaton performing a complex set of coordinated movements once it is set in motion.