One primary practical distinction is that humans initiate spontaneous, unique, non-predetermined actions and machines do not. Advanced machines such as military early warning systems may initiate spontaneous activities, but all of these are predetermined and predefined to a greater or lesser degree. And for any machine responses that are not previously designated and programmed as routine, human interaction is required for initiation of the suggested action.
Generally, even the "smartest" machine only has access to a built-in suite of responses. Noted exceptions include computer programs based on simple rules which emulate living systems. The "cellular automata" generated by these programs demonstrate spontaneous emergent behaviors not predictable from the simple rules themselves. The "artificial life" thus observed is exceedingly complex and can even provide optimal solutions to deep scientific problems posed to the system by investigators.
Cellular automata are examples of complex systems, defined as systems which are critically sensitive to small changes in initial conditions. Complex systems achieved fame in Michael Crichton's novel (and the subsequent Stephen Spielberg film), "Jurassic Park". In the book, the paleobotanist Dr. Ian Malcolm uses the weather as an example of the nature of complex systems. In a scene which has achieved iconic status, Malcolm relates how a butterfly flapping its wings in the Amazon rain forest can cause a tornado in the American Midwest. Small changes in a complex system's initial state can lead to substantial unpredictable results in the system as a whole.
The human nervous system is such a complex, dynamic system.1,2,3 Small changes in the nervous system's initial state can result in dramatic outcomes. For example, a spinal misalignment (resulting from an innocuous low back muscle strain) can irritate a spinal nerve (a small change in initial conditions) which can lead to chronic indigestion, allergies, migraine headaches, or even high blood pressure down the road.
Most machines are not complex in this way, but they do require regular maintenance and care. As human beings, we are complex systems by our very nature and the importance of regular maintenance and care is that much greater. A healthy spine and optimally functioning nerve system go very far toward ensuring the health of our body-as-machine and our associated happiness and well-being.