Pierre Gassendi (1592–1655) argued that certainty or necessary truths could not be discovered in science. (A necessary truth is a belief or statement that it would be logically self-contradictory to deny—a necessary truth must be true.) Gassendi argued that all we can know is how things appear, not how they are in themselves. (In other words, we cannot know the hidden qualities of things.) We have no way to reason from what we experience to what has caused our experience, if we have not experienced that cause. Thus, if we have experienced the effect of something, but not the cause itself, we have to admit that we do not know the cause. Nevertheless, we can develop some useful bodies of information about appearances, especially if we augment that knowledge with atomism as a hypothesis.

In Syntagma Philosophicum Gassendi asks if there is any certain criterion to tell truth from falsehood. Clearly, some things are obvious, even to skeptics, such as “the sun is shining.” It is what is concealed from us that causes difficulty: for example, whether the total number of stars is an odd or even number. Things like that can never be known. But, there are other things that are not evident that we can know by “signs.” Our perception of sweat, for instance, is a sign that we have pores in our skin. There are also naturally non-evident things—such as the hidden fire that causes the smoke we see—that we know through indicative signs. While we do not know that the atomic world exists, we can infer it from indicative signs in the world we do perceive. Gassendi thought that it would be needlessly metaphysical to speculate about the property of atoms, such as claiming that they are mathematical. He also insisted that atomic explanations do not apply to the human soul, which he believed was indivisible and immortal, as held by Church doctrine.