Propositional knowledge refers to knowledge that can be expressed in declarative sentences or indicative propositions. It involves knowing that something is the case, such as "The sky is blue" or "Water boils at 100 degrees Celsius." This type of knowledge is often contrasted with procedural knowledge, which is about knowing how to do something. In philosophy, propositional knowledge is typically defined as justified true belief. This means that for someone to truly know a proposition, it must be true, they must believe it, and they must have justification for that belief. This concept is central to discussions in epistemology, the study of knowledge.