Butterfly effect, and as a result of it the theory that whatever we do is the result of chemical reactions happening inside of us. That means a person's behavior can be simulated given all of his state variables, which may be millions in number. So basically, nothing is random; everything in this universe can be predicted.

Here's a recent AskReddit thread on same lines

https://www.reddit.com/r/AskReddit/comments/779sy4/what_theo...

Bell’s theorem.

https://en.wikipedia.org/wiki/Bell%27s_theorem

Darwin's Theory of Evolution by Natural Selection.