turtles all the way down
expressionprogrammingcryptography
The recursion joke applied to trust: every layer rests on another layer, and eventually you must simply stand somewhere.
PKI ends at self-signed root CAs, DNS at the root servers, secure boot at a burned-in key - every chain of trust bottoms out in an anchor taken on faith. Naming the turtles is how you find out what you are actually trusting.
Turtles all the way down comes from an anecdote about cosmology: the world rests on a turtle, and that turtle rests on another, and it is turtles all the way down. In computing it describes an abstraction stack with no obvious bottom.
A running program sits on a runtime, on an operating system, on a hypervisor, on firmware, on microcode, on silicon that is itself an abstraction over physics. Every layer presents a simplified model of the one below and every layer is leaky, which is why a problem can originate at a depth nobody in the room is equipped to consider.
The honest version acknowledges that the regress does terminate, and where it terminates matters. Trust bottoms out in hardware you did not fabricate, running microcode you cannot read, made by a supply chain you cannot audit, which is exactly the anxiety that Thompson's trusting trust argument formalizes. The practical response is not paralysis but knowing which turtle you are standing on: a debugging session goes badly when the participants have different assumptions about how far down the problem could be.