This paper is an enquiry into the philosophical fault-line that leads from mechanicism to posthumanism. I focus on a central aspect of posthumanism: the erosion of the distinction between organism and machine, nature and art, and the biological and engineering sciences. I claim that this shift can be placed in the seventeenth century, in Descartes's biology. The Cartesian fusion of the natural and technological opened the door to distinctly posthuman understandings of the living body, its relation to technological extensions, and the possibility of its drastic alteration. Descartes's mechanicism demanded a reconceptualization of bodily boundaries, organismic unity, natural finality, causation, and bio/technological instrumentality; all of which Descartes boldly theorized in terms of the wondrous technologies of his day. This radical proposal obscured the possibility of thinking the human as ontologically unique, or as having an ideal unity. This paper will examine the posthuman ramifications of these aspects of Descartes's philosophy.