According to the principle of sufficient reason we should always try to give sufficient reasons for the truth (or high probability) of our opinions. All such attempts lead to a trilemma of justification: they force us to choose between infinite regress, logical circle, or dogmatism. According to the principle of critical testing we should always try to test our opinions critically. It is reasonable to claim that opinions that have survived critical tests are true. Such truth-claims are conjectural and do not confront us with any trilemma of justification. Scientific theories can be tested through observations and experiments. Statements about observations and experiments can be tested by experience. Although Critical Rationalism is a philosophical position, it can also be critically discussed. If we follow the principle of critical testing, no trilemma of justification arises when we claim that it is reasonable to accept Critical Rationalism. This opens the way for a critical philosophy stressing the fallibility of human knowledge.