One of the most exciting issues in psychology is, What are the psychological mechanisms underlying human tool use? The computational approach assumes that the use of a tool (e.g., a hammer) requires the extraction of sensory information about object properties (heavy, rigid), which can then be translated into appropriate motor outputs (grasping, hammering). The ecological approach suggests that humans perceive not the properties of tools per se but what they afford (a heavy, rigid object affords pounding). This is the theory of affordances. In this article, we examine the potential of the computational view and the ecological view to account for human tool use. To anticipate our conclusions, neither of these approaches is likely to be satisfactory, notably because of their incapacity to resolve the issue of why humans spontaneously use tools. In response, we offer an original theoretical framework based on the idea that affordance perception and technical reasoning work together in a dialectical way. The thesis we defend here is that humans have the ability to view body action as a problem to be solved. It is precisely at this point that technical reasoning occurs. However, even if the ability to do technical reasoning gives humans the illusion of constantly doing less (e.g., TV remote control), they are still forced to use body action--and to perceive affordances--to operate the product of the reasoning (pushing buttons with the fingers). This is the principle of dialectic. (Contains 10 footnotes, 1 table, and 4 figures.)