 This reflection on nature and the limits of mechanistic science was accompanied by truly experimental activity. Mersenne was convinced that mathematical sciences of nature cannot settle for common and vague observation. It urgently required “well-controlled and well-performed experiments” (HU I: 167), facts that were artificially generated and precisely measured. Mersenne was aware that, even when rigorously performed, an experiment is not always enough to establish the law of phenomena: two series of experiments can diverge very little and yet express very different laws. Measurement can only be approximated, and this has a bearing on the certainty of mixed mathematics, which is partially compromised to the extent that it deals with physical objects. Reason must therefore, as far as possible, always accompany and discipline observation, without which one is vulnerable to considerable misunderstandings.