P R E F A C E .

do not produce as great a certainty as those of Geometry, and even differ from them greatly; for whereas Geometers prove their Propositions by certain and incontestable Principles, here the Principles are verified by the conclusions drawn from them; the nature of these matters not allowing it to be done otherwise. It is possible, however, to arrive at a degree of probabilityThe original French uses vraisemblance (likelihood). Huygens is describing the "hypothetico-deductive" method, where a theory is proven by how well it explains observations rather than by mathematical first principles. that often yields very little to total certainty. This occurs when the things demonstrated by these assumed Principles relate perfectly to the phenomenaIn this context, "phenomena" refers to observable events in nature, specifically the behavior of light. that experience has noted; especially when there are a great number of them, and even more so when one conceives of and predicts new phenomena that must follow from the hypotheses being used, and finds that the outcome meets our expectations.

If all these proofs of probability are found in what I have proposed to treat—as it seems to me they are—it must be a very great confirmation of the success of my research; and it is unlikely that things are not very nearly as I represent them. I would therefore believe that those who love to know the Causes, and who know how to admire the marvel of Light, will find some satisfaction in these various speculations which