 Most of us think we can safely enlarge our knowledge base by accepting things that are entailed by (or logically implied by) things we know. Roughly speaking, the set of things we know is closed under entailment (or under deduction or logical implication), so we know that a given claim is true upon recognizing, and accepting thereby, that it follows from what we know. This is not to say that our usual way of adding to our knowledge is simply to recognize and accept what follows from what we already know. Obviously much more is involved. For instance, we gather data and construct explanations of those data, and under suitable circumstances we learn from others. More to the point at hand, when we claim that we know, of some proposition, that it is true, that claim is itself subject to error; often, seeing what follows from a knowledge claim prompts us to reassess and even withdraw our claim, instead of concluding, of the things that follow from it, that we know that they are true. Still, it seems reasonable to think that if we do know that some proposition is true then we are in a position to know, of the things that follow from it, that they, too, are true. However, some theorists have denied that knowledge is closed under entailment. The arguments against closure include the following: