 ‘Design’ is the human involvement in the example. In natural philosophy, necessity can be predicated also in another sense. Avempace mentions the case of the carpenter who choses an iron hatchet because of the hardness of its iron; the hatchet as formal cause cannot carry its function unless its matter is iron or an alike material. He sees here another kind of necessity, intrinsec necessity because of matter. Following the example, iron melts by the application of fire or it rusts in the soil. Melting and rusting take place by necessity and “not because the smith intends it” (IB-BA: 81. 16).