I've never taken a rigorous linear algebra class or a multivariable calculus or ODE/PDE class prior to studying cardinalities, injective,surjective, well-ordering principle, minimal counterexample, ZFC. I only know applied single variable calculus and linear algebra for scientists and engineers. Is the reason I'm not understanding how to prove that the real numbers are not countable and surjections and injections between sets and how to use the well ordering principle because I haven't seen how to use the implicit function theorem, how to do curl and divergence, and what's a non-homogeneous equation just to name a few topics from vectorcalculus/ODE/PDE?
Is the reason I'm bad at proving things about the cartesian product, division algorithim, injective and surjective on a characteristic function, powersets, unions and intersections, convex hulls, countability of rationals and integers, partitions of a set, event spaces infinite sequences product because I've never seen ODEs,PDEs, differential geometry?
Many of the people I know who are not struggling with proving things have taken multivariable calculus, vector calculus, ODE/PDE. I'm sure a genius would not struggle with writing proofs even if they never learned how to do ODE/PDE but for the average person, would not knowing ODE/PDE/Vector calculus mean that they haven't practiced or been taught the mathematical maturity needed to prove that the rationals are countable?