I reprint it here because of this and other factors. What we're talking about here are historically related words. When we know we've got cognates, we can talk about reconstruction.
Reconstruction revolves around the notion that sound change is mechanical and exceptionless.
If there are exceptions, there must be some other conditioning factor. So we have some real-life models on which to build our guesses. And semantic change is a really neat thing to trace, though much slipperier than sound change.