  Even if facts could be known with certainty by observation, Pascal conceded that it would require the use of reason to understand or explain natural phenomena, and that “the secrets of nature are hidden” (Preface to the Traité du vide: I, 455). To penetrate those secrets, one has to have recourse to hypotheses. Pascal's account of how hypotheses are confirmed, and the degree of certainty that one could claim for them, were ambivalent. When reflecting on the results of the puy-de-Dôme experiment, he argued against critics that not only had he established that the mercury rises in a barometer because of the weight of the air, but that the empty space at the top of a barometric tube is a vacuum. Others (including Descartes) accepted the experimental results, but disputed this interpretation of them. They agreed that mercury is supported in a barometer by the weight of the atmosphere; they also argued, however, that the apparently empty space in a barometric tube contains a subtle matter of some kind, that it has physical properties (for example, a specific size, or the capacity to be penetrated by light, etc.), and that their interpretation of the apparent vacuum was as consistent as Pascal's with the experimental results.