 The Port-Royal Logic includes eight of Aristotle’s original thirteen fallacies, several of them modified to fit the bent to natural philosophy rather than dialectical argumentation. Several kinds of causal errors are considered under the broad heading, non causa pro causa and they are illustrated with reference to scientific explanations that have assigned false causes for empirical phenomena. Also identified as a common fallacy of the human mind is post hoc, ergo propter hoc: “This happened following a certain thing, hence that thing must be its cause” (Bk. III, xix 3). Begging the questions is included and illustrated, interestingly, with examples drawn from Aristotelian science. Two new sophisms are included: one is imperfect enumeration, the error of overlooking an alternative, the other is a faulty (incomplete) induction, what we might call hasty generalization. Although the discussions here are brief, they mark the entry of inductive fallacies into the pool of present day recognized fallacies. Ignoratio elenchi retains its dialogical setting but is extended beyond the mere failure to contradict a thesis, “to attribut[ing] to our adversaries something remote from their views to gain an advantage over them, or to impute to them consequences we imagine can be drawn from their doctrines, although they disavow and deny them” (Bk. III, xix 1). The other Aristotelian fallacies included are accident, combination and division, secundum quid and ambiguity.