I'm thinking about the phrase of David Hume that suggest that causality fills the world of beings. I don't know how to understand that. This phrase indicates that causality makes experiences possible ? Indeed the principle of causality cannot be justified rationally, so that the custom, in Hume's theory, constitutes the foundation of the causal relation. It is the experience, the constant conjunction and the memory, which makes our inference possible of A to B. With regard to the paragraph on the existence of bodies Hume also it pushes the rational justification back, my question is the following one: if the causality fills to the world of beings or "Tis this latter principle which peoples the world": must I understand that the causality allows us to have experiences? but if the experience is the foundation of the causality: how can the experience be a foundation and result of the causality?
This extract from Treatise, I.iii.9, is not Hume's most lucid passage but let's get the whole of it in view before trying to work out what he intended to convey:
It is evident, that whatever is present to the memory, striking upon the mind with a vivacity, which resembles an immediate impression, must become of considerable moment in all the operations of the mind, and must easily distinguish itself above the mere fictions of the imagination. Of these impressions or ideas of the memory we form a kind of system, comprehending whatever we remember to have been present, either to our internal perception or senses; and every particular of that system, joined to the present impressions, we are pleased to call a reality. But the mind stops not here. For finding, that with this system of perceptions, there is another connected by custom, or if you will, by the relation of cause or effect, it proceeds to the consideration of their ideas; and as it feels that it is in a manner necessarily determined to view these particular ideas, and that the custom or relation, by which it is determined, admits not of the least change, it forms them into a new system, which it likewise dignifies with the title of realities. The first of these systems is the object of the memory and senses; the second of the judgment.
It is this latter principle, which peoples the world, and brings us acquainted with such existences, as by their removal in time and place, lie beyond the reach of the senses and memory. By means of it I paint the universe in my imagination, and fix my attention on any part of it I please. I form an idea of ROME, which I neither see nor remember; but which is connected with such impressions as I remember to have received from the conversation and books of travellers and historians. This idea of Rome I place in a certain situation on the idea of an object, which I call the globe. I join to it the conception of a particular government, and religion, and manners. I look backward and consider its first foundation; its several revolutions, successes, and misfortunes. All this, and everything else, which I believe, are nothing but ideas; though by their force and settled order, arising from custom and the relation of cause and effect, they distinguish themselves from the other ideas, which are merely the offspring of the imagination.
I understand Hume to be be saying the following.
There is a realm - of 'system' - of personal experience comprised of memory and of impressions such as my remembering an event that happened last week (an idea) and my having the current sensation of a red patch in my visual field or of a piercing pain in my left knee (an impression). When certain regularities occur between our impresssions, or between our impressions and our ideas, or between our ideas, we read the world causally; we project causal relations onto our experience.
This is just how 'human nature' is. But if our knowledge and belief were confined to personal experience in the sense of these happened-to-me examples (of memory and [the] senses into which causality fits), they would be extremely limited. Our experience would constitute a bloc of data which 'admits not of the least change' at any given time; and since knowledge and belief of 'matters of fact' (as distinct from 'relations of ideas' = roughly, maths and logic) derive from experience, we could never in our knowledge and belief pass beyond whatever bloc of data we happened at any given time to have.
But our epistemological predicament is not so dire, so constricted to this first 'system'. This is because Hume admits 'such impressions as I remember to have received from the conversation and books of travellers and historians'. From these impressions I can pass beyond the kind of happened-to-me experiences listed in the first paragraph to the exercise of judgment, a different 'system'. I can infer to the existences of objects, events and states of affairs, on the basis of (to repeat) 'such impressions as I remember to have received from the conversation and books of travellers and historians'. By the use of judgment I can infer the existence of Ancient Rome, specifically geographically located in a region I have never visited, and embodying (if we specify an era in the city's history) 'a particular government, and religion, and manners'.
Judgment has nothing directly to do with causality, which fits into the first system - and not the second 'system, or as Hume calls it, 'the 'latter principle'.
Where Hume talks of the books of travellers and historians, the scope of judgment is by the strongest implication much wider than historical matters, which are merely illustrative. We can add the books of 'natural philosophers' (scientists) and of indefinitely many others.