@yoshi, your dad is right. He has made the leap from using numbers to count things to using them to represent ideas. Students are first exposed to the potential for this when negative numbers are introduced and then when rational numbers like a 1/2 are included. I'm reasonably certain that the majority of mathematics teachers in the UK at junior school don't even realise that there is a fundamental shift from counting things to the abstraction of describing the location of points on a line taking place. They talk about the "number line" without getting that it is a major leap.
The next leap takes place when you want to describe points on a plane. A single number isn't enough and a coordinate system is just slipped in without the fanfare it deserves. There are multiple ways to set up the coordinate system for a plane and relationships between them, this is usually not even mentioned
Mathematics is taught in little boxes and the chance to have pupils think on a grander scale is missed. By the time students are 17 or older it is now quite hard to not think in little boxes. Counting has become automatic, ingrained, second nature.
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Is a plane geometry potentially useful? If yes how do we make use of it? This is the same as asking how do you perform calculations upon points on a plane . Answering these questions will inevitably lead to the invention/discovery of root(-1) as a valuable idea. That it can be used to glue together different areas of mathematical thinking into something both simpler and more grand is what makes it beautiful.
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JMO but much of the problem with the understanding of mathematics is that it is taught in compartments often by teachers with very little training in mathematics. I was shocked to find out that there wasn't a single teacher in any of the schools that my sons attended up to the age of 11 that had any formal qualification in mathematics, other than the exams they took when they were pupils themselves, sciences were similarly absent. Given the latter it isn't at all surprising that very big shifts in mathematical thinking are often unexplored. unexplored if only because of the needs of the curriculum and its boxes of skills that pupils have to learn without any requirement to understand. That those really big shifts are often very subtle doesn't help either.
counting --> points on a line --> points in space.
I think I was in my very late teens when I started to truely let go of counting and to treat numbers as abstractions. I'd been exposed to imaginary numbers one way and another for around 6 years at that point. Now I look back and can't really think why I held on to counting for so long.