We propose a logic for specifying and proving properties of membrane systems. The main idea is to approach a membrane system by using the “point of view” of an external observer. Observers (as epistemic agents) accumulate their knowledge from the partial information they collect by observing subparts of the system and by applying logical reasoning to this information. We provide a formal framework to combine and interpret distributed knowledge in order to recover the complete knowledge about a membrane system. The proposed logic can be used to model biological situations where information concerning parts of the biological system is missing or incomplete.