these "unphilosophical" influences on our reasoning. Sections 14–16 Having completed his account of probable reasoning, Hume returns to the mysterious idea of necessary connection. He rejects some proposed sources of this idea: not from the "known qualities of matter", nor from God, nor from some "unknown quality" of matter, nor from our power to move our body at will. For all ideas derive from experience, and in no single case do we observe anything like a necessary connection linking cause to effect. But the idea does arise upon repeated observations, and since mere repetition cannot produce anything new in the objects themselves, the idea must therefore derive from something new in our mind. Thus he concludes that the idea of necessary connection is derived from inside: from the feeling we experience when the mind (conditioned by repeated observation) makes a causal inference. And though his conclusion is shocking to common sense, Hume explains it away by noting that "the mind has a great propensity to spread itself on external objects". Finally, he offers two definitions of "cause": one in terms of the objects (viz. their relations of priority, contiguity, and constant conjunction), and another in terms of the mind (viz. the causal inference it makes upon observing the objects). Hume finishes Part 3 with two brief sections. First, he presents eight rules for empirically identifying true causes: after all, if we leave aside experience, "[a]ny thing may produce any thing". Second, he compares human reason with animal reason, a comparison which clinches the case for his associationist account of probable reasoning: after all, animals are clearly capable of learning from experience through conditioning, and yet they are clearly incapable of any sophisticated reasoning. Part 4: Of the sceptical and other systems of philosophy Sections 1–2 Hume begins Part 4 by arguing that "all knowledge degenerates into probability", due to the possibility of error: even the rock solid certainty of mathematics becomes less than certain when we remember that we might have made a mistake somewhere. But things get worse: reflection on the fallibility of our mind, and meta-reflection on the fallibility of this first reflection, and so on ad infinitum, ultimately reduces probability into total scepticism—or at least it would, if our beliefs were governed by the understanding alone. But according to Hume, this "extinction of belief" does not actually happen: having beliefs is part of human nature, which only