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In constructor theory, physical laws are formulated only in terms of which tasks are possible (with arbitrarily high accuracy, reliability, and repeatability), and which are impossible, and why – as opposed to what happens, and what does not happen, given dynamical laws and initial conditions. A task is impossible if there is a law of physics that forbids it. Otherwise, it is possible – which means that a constructor for that task – an object that causes the task to occur and retains the ability to cause it again – can be approximated arbitrarily well in reality. Car factories, robots and living cells are all accurate approximations to constructors.


For constructors that survive for long, information in the recipe must be digital, to make reliable error correction possible after copying: if not, there would be a fundamental limit to how well an error can be detected, which would lead to a build up of errors and a limit to the accuracy and resiliency achievable. A self‑reproducing cell must do all this, too. The parent cell contains a recipe – DNA – with all the instructions to construct a new cell (recipe excluded). This means that accurate self‑reproduction can occur only in two steps. Using letter-by-letter replication and error-correction, the parent cell makes a high-fidelity copy of the recipe to be inserted in the new cell; then it constructs the copying mechanism plus the rest of the cell afresh, following the recipe. It was the Hungarian-born physicist John von Neumann who first discovered this logic in the 1940s. He was exploring cellular automata – discrete computational models used, for instance, in Conway’s Game of Life, which rely on unphysical dynamical laws. Constructor theory shows that this is the only possible logic for accurate self-reproduction given any no-design laws.

Constructor theory gives the ‘recipe’ an exact characterisation in fundamental physics. It is digitally coded information that can act as a constructor and has resiliency – the capacity, once it is instantiated in physical systems, to remain so instantiated. In constructor theory, that is called knowledge – a term used here without the usual connotation that it is known by someone: it merely denotes this particular kind of information with causal power and resiliency. And an essential part of the explanation of all distinctive properties of living things (and of accurate constructors in general) is that they contain knowledge in that sense.

Moreover, it is a fundamental idea of constructor theory that any transformation that is not forbidden by the laws of physics can be achieved given the requisite knowledge. There is no third possibility: either the laws of physics forbid it, or it is achievable. This accounts for another aspect of the evolutionary story. Ever better constructors can be produced, without limit, given the relevant knowledge, instantiated in digital recipes.

The early history of evolution is, in constructor-theoretic terms, a lengthy, highly inaccurate, non-purposive construction that eventually produced knowledge-bearing recipes out of elementary things containing none. These elementary things are simple chemicals such as short RNA strands, which can perform only low-fidelity replication, and so do not bear the appearance of design, and are therefore allowed to exist in a pre-biotic environment governed by no-design laws.

Thus the constructor theory of life shows explicitly that natural selection does not need to assume the existence of any initial recipe, containing knowledge, to get started. It shows that, whatever recipes we might find in living things, they do not require ad‑hoc, biocentric or mysterious laws of physics in order to come into existence from elementary initial components. They need only the laws of physics to permit the existence of digital information, plus sufficient time and energy, which are non-specific to life. This adds another deep reason why a unification in our understanding of the phenomena of life and physics is possible. Whatever the laws of physics do not forbid us, we can do. Whether or not we will, depends on how much knowledge we create. It is up to us.

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