I assume that the readers know the Bayes' rule already. If you are not familiar to it, read any kind of textbook about probability, data science, and machine learning. I recommend the book, which I learned Bayes' rule. Bayesians say that you cannot do inference without making assumptions. Thus, Bayesians also use probabilities to describe inferences. The author in the chapter 2 introduces some rules of probability theory and introduces more about assumptions in inference in the chapter 3.
I announce over and over that the chronicle ordering of the post are irrelevant for beginners' favor. There are many blanks I skipped. I would fill the holes later. Variational method During my physics coursework and researches, I used this method countlessly. I even had a book of the name. It is quite simple, but also as big topic as being a book. Simply put, it is a technique to find equations and solutions (sometimes approximate solutions) by extremizing functionals which is mainly just integrals of fields, and treat the functions in the integral, as parameters.