One of the amazing things about computer science is that the physical implementation is in some sense "irrelevant". People have successfully built computers out of several different substrates -- relays, vacuum tubes, discrete transistors, etc. People may soon succeed in building Turing-complete computers out of non-linear optical materials, various biomolecules, and a few other substrates. In principle, it seems possible to build a billiard-ball computer.
However, the physical substrate is not completely irrelevant. People have found that certain sets of components -- in particular, diode-resistor logic -- are "incomplete": no matter how many of them you connect to a power supply and to each other, there are certain very simple things that it cannot do. (The diode-resistor logic can implement AND, OR, but fails to implement NOT). Also, certain ways of connecting components -- in particular, single-layer perceptrons -- are "incomplete": there are certain very simple things that they cannot do. (A single-layer perceptron can implement AND, OR, NOT, but fails to implement XOR).
Is there a less-awkward phrase for "physical things out of which one can build a Turing machine"? Or for the opposite, "physical things that, no matter how many of them one has, cannot form a Turing machine"?
For a while I used the phrase "functionally complete set" or "universal set of gates" -- or, when speaking to mathematicians, "physical things that can implement a functionally complete set" -- but I've been told that isn't quite correct. Some sets of components can implement a functionally complete set; and yet it is not possible to build a Turing-complete machine entirely out of these components. For example, light bulbs and manually-operated 4-way light switches can implement a functionally complete set (AND, OR, NOT, XOR, etc.); and yet it is not possible to build a Turing-complete machine entirely out of light switches and light bulbs, since the (electrical or optical) output of one cannot be fed into the (mechanically rotating) input of the next.