A countable Boolean algebra that is Reichenbach's common cause complete
Dominika Burešová
TL;DR
The paper addresses whether common cause completeness (CCC) can hold in a small, countable Boolean algebra. It constructs a countable interval-algebra $B$ of finite unions of rational-endpoint sub-intervals of $(0,1]$ with $p$ as the Lebesgue measure restricted to $B$, and shows that for any $a,b$ with $ ext{cov}_p(a,b)>0$ there exists a common cause $c$ with $p(c)=v$ where $v=rac{p(a\land b)-p(a)p(b)}{1+p(a\land b)-p(a)-p(b)}>0$, with $c\le a\lor b$, thereby establishing non-trivial CCC. The same construction idea yields an uncountable CCC example, and the authors discuss extensions to quantum logics via horizontal-sum constructions. A key takeaway is that CCC can arise in countable Boolean algebras without requiring $\sigma$-completeness or a non-atomic measure, providing a simple, explicit counterpoint to prior examples.
Abstract
The common cause completeness (CCC) is a philosophical principle that asserts that if we consider two positively correlated events then it evokes a common cause. The principle is due to H. Reichenbach and has been largely studied in Boolean algebras and elsewhere.The results published so far bring about a question whether there is a small (countable) Boolean algebra with CCC. In this note we construct such an example.
