Written over a year ago, but only just coming to my attention, is Google engineer Jean-Baptiste Quéru's wonderful essay describing how no single person alive understands entirely what's going on in the machines we use daily.
You just pressed a key on your keyboard.
Simple, isn't it?
What just actually happened?
Well, when you know about bit about how input peripherals work, it's not quite that simple. You've just put into play a power regulator, a debouncer, an input multiplexer, a USB device stack, a USB hub stack, all of that implemented in a single chip. That chip is built around thinly sliced wafers of highly purified single-crystal silicon ingot, doped with minute quantities of other atoms that are blasted into the crystal structure, interconnected with multiple layers of aluminum or copper, that are deposited according to patterns of high-energy ultraviolet light that are focused to a precision of a fraction of a micron, connected to the outside world via thin gold wires, all inside a packaging made of a dimensionally and thermally stable resin. The doping patterns and the interconnects implement transistors, which are grouped together to create logic gates. In some parts of the chip, logic gates are combined to create arithmetic and bitwise functions, which are combined to create an ALU. In another part of the chip, logic gates are combined into bistable loops, which are lined up into rows, which are combined with selectors to create a register bank. In another part of the chip, logic gates are combined into bus controllers and instruction decoders and microcode to create an execution scheduler. In another part of the chip, they're combined into address and data multiplexers and timing circuitry to create a memory controller. There's even more. Those are actually such incredibly complex technologies that they'll make any engineer dizzy if they think about them too much, and such that no single company can deal with that entire complexity.
Can we simplify further?
In fact, very scarily, no, we can't. We can barely comprehend the complexity of a single chip in a computer keyboard, and yet there's no simpler level. The next step takes us to the software that is used to design the chip's logic, and that software itself has a level of complexity that requires to go back to the top of the loop.
Today's computers are so complex that they can only be designed and manufactured with slightly less complex computers. In turn the computers used for the design and manufacture are so complex that they themselves can only be designed and manufactured with slightly less complex computers. You'd have to go through many such loops to get back to a level that could possibly be re-built from scratch.
Quéru's description of basic complexity reminds me of descriptions of the cell and the atom - that even when we break things down to what we feel is the fundamental level, there's still so much going on that we can scarcely understand it. And moreover, that just like living, evolved organisms, each computer is the offspring of a slightly less complicated computer, a continuous chain whose links cannot stand alone any more than life can spontaneously arise in a sealed jar. A tangled bank indeed...