>> I don't think the invention of calculus was hard.
If you look at it with ~2k years of hindsight and mathematical knowledge, sure, it wasn't hard. Neither was it "hard" to think of using zero or infinity, or even the unit.
>> When it was needed it was promptly developed twice on the same continent and at the same time.
Most probably the right way to see this is completely the opposite: calculus was needed countless times before and it was only developed twice.
If you look at it with ~2k years of hindsight and mathematical knowledge, sure, it wasn't hard. Neither was it "hard" to think of using zero or infinity, or even the unit.
>> When it was needed it was promptly developed twice on the same continent and at the same time.
Most probably the right way to see this is completely the opposite: calculus was needed countless times before and it was only developed twice.