confirms Hume's account of belief as "more properly an act of the sensitive, than of the cogitative part of our natures". And as for why we do not sink into total scepticism, Hume argues that the mind has a limited quantity of "force and activity", and that difficult and abstruse reasoning "strain[s] the imagination", "hinder[ing] the regular flowing of the passions and sentiments". As a result, extremely subtle sceptical argumentation is unable to overpower and destroy our beliefs. Next comes an extremely lengthy account of why we believe in an external physical world: i.e., why we think objects have a continued (existing when unobserved) and distinct (existing external to and independent of the mind) existence. Hume considers three potential sources of this belief—the senses, reason, and the imagination. It's not the senses: clearly they are incapable of informing us of anything existing unobserved. Nor can they inform us of objects with distinct existence: the senses present us with sense perceptions only, which means they cannot present them as representations of some further objects, nor present them as themselves objects with distinct existence (for the senses are unable to identify the mysterious self, distinguishing it from and comparing it with sense perceptions). And it's not reason: even children and fools believe in an external world, and nearly all of us naïvely take our perceptions to be objects with a continued and distinct existence, which goes against reason. So this belief must come from the imagination. But only some of our impressions bring about the belief: namely, impressions with constancy (invariableness in appearance over time) and coherence (regularity in changing appearances). Thus Hume proceeds to develop an account of how the imagination, fed with coherent and constant impressions, brings about belief in objects with continued (and therefore distinct) existence. Given coherent impressions, we have only one way of accounting for our observations consistently with past experience: we form the supposition that certain objects exist unperceived. And since this supposes more regularity than is found in past observation, causal reasoning alone cannot explain it: thus Hume invokes the imagination's tendency to continue in any "train of thinking" inertially, "like a galley put in motion by the oars". But to explain "so vast an edifice, as... the continu'd existence of all external bodies", Hume finds it necessary to bring constancy into his account, as follows: (1) Identity is characterised as invariableness and uninterruptedness over