 To sketch the necessity argument, we must introduce a bit of terminology. An agent’s degree of entrenchment for a proposition \(A\) is the number of “independent and minimally positively reliable” (mp-reliable) information sources saying \(A\) that it would take for the agent to give up her disbelief that \(A\). If the agent does not disbelieve \(A\) to begin with, her degree of entrenchment for \(A\) is 0. If no finite number of information sources is able to make the agent give up her disbelief that \(A\), her degree of entrenchment for \(A\) is \(\infty\). Of course, information sources we typically encounter are rarely independent or mp-reliable. However, this is not crucial. Independent, mp-reliable sources are a theoretical construct – the connection between entrenchment and update is stated as a counterfactual because the agent need never actually encounter such information sources.