 An aspect that has so far been largely neglected in this debate is a different kind of robustness, namely physical robustness. If we compare the case of the mesosomes with that of other intracellular organelles, for example, the eukaryotic nucleus or mitochondria, it is notable that it was never possible to isolate mesosomes, whereas the latter organelles can be recovered in more or less intact form from cell extracts, normally by the use of centrifugation. Although it is, of course, entirely possible that the structures so recovered are artifacts of the preparation procedure, there may be additional evidence for their reality. For example, there could be telltale enzyme activities that can be measured in isolated cell organelles that correspond to the organelle’s function. It might be possible to subsume this kind of reasoning under Hacking’s (1983) strategies of using the ability to intervene with theoretical entities as a ground for their real existence.