So the example holds because when p entails q, it means that q is included in the very meaning of p. Negative statements often make claims that are hard to prove because they make predictions about things we are in practice unable to observe in a finite time. In such a case we have proved a negative, just not the negative of the sweeping proposition in question. We can only infer it. Or, in the case of propositions for which we have abundant but incomplete proof, we recognize that it may be false, but have insufficient grounds to disbelieve it.
Unprovable Statements Consider the negative case.
Thus, when finally deciding what to believe, it is clear that the best policy is to assume that all unprovables are false, until such time as they are proved. In other words, it is reasonable to disbelieve a proposition when there is no evidence. A similar line of reasoning establishes the opposite in all positive cases. Of course, it would be plainly absurd to believe that all the statements for which we have some evidence are false. Thus, when finally deciding what to believe, it is clear that the best policy is to assume that all unprovables for which we have good evidence are true, until such time as they are disproved.