But even on the continuum, there will be usually areas. If you consider the trolley problem and you're dealing with a threshold deontologist, having zero persons on the track will likely result ("I don't throw the guy over the bridget", hopefully) in a different answer then having one person on the track, and at some point the answer will shift again. We'll only need to reduce the problem to find the points of change (or areas, as few people will probably say "well, at 99? no, but for a 100 lives, yeah, I'll throw him over"), we don't need to keep a 1:1 mapping of all possible input values with the output values.

To use things, we usually reduce the single parts and use abstractions to deal with what's important (to us in that context). You don't deal with individual atoms, you look at clusters and clusters of clusters etc and understand and use them alltogether as a pen, just like you've similar pens before, even though they were 100% completely different if you look close enough.