 In the case of non-philosophical questions, even if the answer is unknown, the means for discovering the answer is known, or accepted, by most people. Thus questions of empirical fact can be answered by observation. Other questions can be answered deductively, by referring to established rules: this is the case, for example, with mathematics, grammar and formal logic. For example, even if we do not know the solution to a difficult mathematical problem, we do know the rules and techniques that should lead us to the answer.