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# 4 of Swords: The Halting Problem

### A razor for the barber who shaves everyone that does not shave themselves

Computers are “universal machines”, in that one computer can simulate any other. The suit of swords represents the reflexive quality of computing.

The fourth card in a tarot suit represents the shock of that suit’s core quality, dropped out of the ideal and into the chaos of the real world. The motivating crisis of the universal, reflexive quality of computing can be seen in **the halting problem:** because computers can simulate one another, we can prove that certain kinds of very useful programs are impossible to write.

When Alan Turing wrote the paper that described the halting problem, he also produced what we think of as the first description of a modern computer. In the service of proving that some programs were impossible, he had to invent programming.

The details of the halting problem are complex for this short message, but not too complex for non-specialists to understand. Turing’s paper has a scary technical title, but like a lot of foundational science papers, it is relatively easy for non-specialists to read because there weren’t any specialists around when it was written. It’s worth a look.

How are constraints structuring your thinking? How can what is impossible inform what is possible?