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Before Turing Machines, computability theory, and computational complexity theory, there were computers. They were people: mostly women. This is one of my favorite photos of a computer, because it shows her using two distinct computing methods.

The other is analog: using a slide rule. Here, numbers are represented by a physical magnitude; in this case, length. So rather than π being "3.14159", it is a length of 3.141 units on the slide rule. Analog computing amounts to manipulating these magnitudes, rather than individual digits.

Both methods have been used for millennia, and both have been mechanized. Computing machines were once primarily analog, but now they're almost exclusively digital; we just call those machines computers now. Importantly, they're both real types of computation, although it's easy to forget.

Why? Because Turing (and others) were interested only in digital computation, or "effective procedures" when it came to studying what is "computable." They didn't mean computable by *any* device: they meant to capture "computable by hand, following simple rules, without cleverness or instruments."