 The first three rules define what it is to be cause and effect: as we have seen, they must be contiguous in space and time; the cause must be a prior to the effect; there must a constant union between cause and effect (THN 1.3.15.3–5). With the exception of these first three (see  section 4.4),  Hume’s rules of reasoning have a strong resemblance to Newton’s four “Rules for the Study of Natural Philosophy” outlined at the start of Book 3 of the Principia (third edition). For example, Hume’s fourth rule, “The same cause always produces the same effect, and the same effect never arises but from the same cause”, is explained by 