Probability theory. It is closely aligned to my intuition now, but when I first learned it, it was difficult to accept beyond manipulating formulae. Linearity

Probability theory. It is closely aligned to my intuition now, but when I first learned it, it was difficult to accept beyond manipulating formulae. Linearity aka most of linear algebra. Again, beyond manipulating formulae, many concepts eventually become intuitive with enough application, but its a hard won intuition to acquire.