Perhaps, roughly, I might be described as advanced undergraduate regarding mathematics. However, I have not learned statistics and have only learned elementary probability. Does there exist a book or monograph that introduces probability and statistics at this level and still covers frequentist and bayesian views (philosophy?) in a balanced manner?
It appears to me (but please correct me if I am wrong -- as I have stated I haven't learned this yet) that introductions at this level usually fully adopt a frequentist view and don't really broach the subject. On the other hand, bayesian books appear to be pitched to a more experienced audience and/or are perhaps even more unbalanced, in the sense that they seem anti-frequentist as much as pro-bayesian.
To more fully describe my mathematical maturity, I am comfortable with the normal calculus sequence (although somewhat rusty), basic linear algebra, basic set theory, mathematical logic, computability theory, some abstract algebra, very little category theory. I am comfortable with the level of introductory analysis, but have not completed it, and I am not versed in measure theory (I expect I could handle measure theory, but knowledge of it shouldn't be assumed). I am often interested in foundational topics and a philosophical viewpoint, and for example I particularly enjoy reading Peter Smith (e.g. An Introduction to Gödel’s Theorems).
I would recommend the following 16-page article from 36 years ago, which is easily accessible to any upper-level undergraduate:
This won an MAA Writing Award.
In my inexpert opinion, it is quite unsettling. It does not offer a resolution.
Edit: The above-mentioned article impressed me when I was a student. But there has been much follow-up. For example:
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Further edit: Oops, my ignorance is showing.
B. Efron was president of the ASA in 2004; see his presidential address "Bayesians, Frequentists, and Scientists" at https://efron.ckirby.su.domains/papers/2005BayesFreqSci.pdf.
He won the 2014 Guy Medal in Gold.
See also his CV (WaybackMachine) and this previous Math.SE Answer about his work on bootstrapping, etc.