Consider an automated factory for producing goods to a particular specification. Though its purpose may only be to produce those goods, the laws of physics imply that it must transform something into them and, typically, also use other resources and produce waste products. Very few such transformations happen spontaneously; that is to say, almost all require a constructor, which I shall define as anything that can cause transformations in physical systems without undergoing any net change in its ability to do so. I shall call those physical systems the constructor’s substrates:
input state of substrate(s) output state of substrate(s) . (1)
A transformation, regarded as being caused by a constructor, I call a construction.
"everything physical is composed of elementary constituents such as particles, fields and spacetime; there is
an initial state of those constituents; and laws of motion determine how the state
evolves continuously thereafter."
" a construction (1) is characterized only
by its inputs and outputs, and involves subsystems (the constructor and the
substrate), playing different roles, and most constructors are themselves composite