camp—adding on a sceptical note that his "intention never was to penetrate into the nature of bodies, or explain the secret causes of their operations", but only "to explain the nature and causes of our perceptions, or impressions and ideas". In the final section, Hume accounts for our ideas of existence and of external existence. First, he argues that there is no distinct impression from which to derive the idea of existence. Instead, this idea is nothing more than the idea of any object, so that thinking of something and thinking of it as existent is the very same thing. Next, he argues that we cannot conceive of anything beyond our own perceptions; thus our conception of the existence of external objects is at most a "relative idea". Part 3: Of knowledge and probability Sections 1–3 Hume recalls the seven philosophical relations, and divides them into two classes: four which can give us "knowledge and certainty", and three which cannot. (This division reappears in Hume's first Enquiry as "relations of ideas" and "matters of fact", respectively.) As for the four relations, he notes, all can yield knowledge by way of intuition: immediate recognition of a relation (e.g., one idea as brighter in colour than another). But with one of the four, "proportions in quantity or number", we commonly achieve knowledge by way of demonstration: step-by-step inferential reasoning (e.g., proofs in geometry). Hume makes two remarks on demonstrative reasoning in mathematics: that geometry is not as precise as algebra (though still generally reliable), and that mathematical ideas are not "spiritual and refin'd perceptions", but instead copied from impressions. As for the other three relations, two of them (identity and space/time) are simply a matter of immediate sensory perception (e.g., one object next to another). But with the last relation, causation, we can go beyond the senses, by way of a form of inferential reasoning he calls probable reasoning. Here Hume embarks on his celebrated examination of causation, beginning with the question From what impression do we derive our idea of causation? All that can be observed in a single instance of cause and effect are two relations: contiguity in space, and priority in time. But Hume insists that our idea of causation also includes a mysterious necessary connection linking cause to effect. "[S]topt short" by this problem, Hume puts the idea of necessary connection on hold and examines two related questions: