Earlier quoted context omitted.
By that logic you would need to call like half of all useful phenomena that most people's lives consist of and depend on, as "man-made delusion". We still need to interact with our lives, and many of the abstractions help us do that. BTW if you just replace the word "delusion" with "abstraction" you'll probably see how it makes a lot of sense. Every abstraction is a delusion by definition, is it not? Because the abst…
Are they really useful or are they just easy to copy and to rember? Staying power of memes in culture has little to do with their usefulness. Whole lot of popular narratives distract us from the nature of reality rather than helps us interact with it. I would gladly see introduction to particle physics that doesn't show a single sharp rigid sphere.
Many such distinctions that cannot be made rigorous that stick in various cultures but never existed in others simply exist because a man copies his fellow man, not because they are useful in achieving any goals.
Though, as I said, they exist to simplify reality. I assume he human brain works so it does, simply to conserve energy and admit an inaccurate though usually useful enough solution. The problem is when such fuzzy logic is mixed with actual rigorous logic and attempts are made to analyse the former with the latter — that I find a futile waste of time.
There is a very good reason that no clear answer has been found to the quæstion raised in The Ship of Theseus even after millennia of contemplation — there is none, and the quæstion is bereft of sense.