Testability as you describe it seems to give you more than just knowledge, but also some amount of understanding: understanding of consequences (not necessarily

Testability as you describe it seems to give you more than just knowledge, but also some amount of understanding: understanding of consequences (not necessarily understanding of causes) — you mentioned the ability to make predictions about the consequences of actions (e.g. 'tests'). (Aside: it seems that you can say you know something, it's a narrow enough concept to be sharp, while understanding something can only ever be true to a degree: it's broad without limit!) But you may have conflated 'testability' and 'tested'. Can I know there is a cow in the field if I don't check? Seeing it was already evidence, testing just collects more evidence, so how can that matter? Should we set a certainty threshold on knowledge? Could be reasonable. Maybe prediction-making is too strong to be necessary for 'knowing', if we allow knowing some fact in a domain of knowledge of which you're otherwise clueless. Although very reasonable to not call this knowledge. Suppose I learn of an mathematical theorem in a field that's so unfamiliar that I can't collect evidence to independently gain confidence in it.