 In the process of making this argument, Aristotle also stresses a crucial and problematic dis-analogy: Natural things, once their generation begins, ‘grow by means of themselves’ (735a13–14). This is the most fundamental difference between art and nature for Aristotle; a nature is a source of change within the thing itself (Physics II. 1). Natural development, then, though initiated by the warmth contributed by the male parent, continues by means of a source of change within the developing embryo. The carpenter may not mix any of his own material in his buildings; but he must be continuously, physically involved in the movements that create those buildings. Not so with the male parent in natural generation. So how does a complex process that has its origins in an external generator become a natural process of development?