3Blue1Brown has been recommended on HN before (https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw)

But it's so great I think it bears bringing up in case anyone reading this comment has missed it. This channel animates math proofs, shows you the intuition behind fields of math, and constructs subjects in such a way as to make you feel like you could have invented them yourself. You'll still need a good textbook or something to go with the videos, but they cement the understanding in your mind very well and even open the door to more formal proofs.

My favorite video is https://www.youtube.com/watch?v=NaL_Cb42WyY . You'll have to take an additional step to get a complete proof (hint: read and work through the One Sentence Proof), but this is a beautiful piece of mathematics, shown in an understandable manner.

Bill Shillito's lecture series for Project Polymath is by a mile, the best introduction to higher level mathematics. It requires absolutely no prerequisite knowledge.

Introduction to Higher Mathematics: https://www.youtube.com/playlist?list=PLZzHxk_TPOStgPtqRZ6Kz...

If you find he talks a bit slow, change your playback speed to 1.5x. Enjoy! :-)

There is one incredible one for probability: http://www.randomservices.org/random/

"What is Mathematics?" (https://en.wikipedia.org/wiki/What_Is_Mathematics%3F) is a nice overview.

I had the same thought as I clicked on this. I've thought about it before, too. I wish there was a Feynman Lectures equivalent for math (and other subjects, too). For someone who didn't take much math in undergrad (regrettably), I find it difficult to navigate on my own, especially as it's a very incremental subject in the beginning (i.e., you can't ignore the fundamentals, or you'll be lost/confused).

Although I like https://brilliant.org/, I find some of the lessons to be a little too thin for me.

Funny, I thought the same thing

yes, first thing that came to my mind too

Hi, I'm a professional (applied) mathematician and a website like that for mathematics is what I always dreamt of making ;) Even tried to apply to YC once with the idea but didn't really get very far with it. The need to provide a paycheck for my family unfortunately makes it quite hard to find enough free time to proceed with the plan but maybe if there are enough people here who are interested, we can do it together?

If interested, drop me a message at jerry at millionintegrals com.

I'm sure I read an article once, maybe about Andrew Wiles, where they said something like "there is no map of mathematics, you can trace the routes from point A to point B, but you'll never get a feel for the territory"...