I'm thinking about the phrase of David Hume that suggests 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 [ideas and impressions]; 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.
Personal experience comprises impressions such as my having the current sensation of a red patch in my visual field or of a piercing pain in my left knee (an impression) and my remembering an event that happened last week (an idea of copy of that impression), say my memory of last week's piercing pain. 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. We suppose that one impression causes another, that one idea causes another.
This just is the psychology of human beings, according to Hume, but the heart of your puzzlement and inquiry involves more than causation though the quotations below show that it does include causation.
Hume's main point in the question you raise is that if our belief were confined to personal experience in the sense of only such happened-to-me examples as seeing red and feeling pain [the first system] and the ideas of which they are copies, they would be extremely limited. Our experience would constitute a bloc of data which 'admits not of the least change' at any given time (kind of patently obvious). And if the data changed over time, as it does, it would hardly encompass more than a minute fragment of human experience.
How do we link my fraction or sliver of experience [the first system] to the wider realm of human experience or any major part of it [the second sytem]?
Here's the key, Hume's anyway or Hume's key in my hand. Hume invokes 'such impressions as I remember to have received from the conversation and books of travellers and historians'. This is no part of the first system - the happened-to-me and remembered by me stuff. Through 'such impressions as I remember to have received from the conversation and books of travellers and historians', I can form a belief in the existence of Ancient Rome, 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' [the second system].
How can judgement do this work? The answer is strongly connected with Hume's account of belief. A belief is an idea we assent to; and seen as Hume regards it a judgement is a 'lively' 'solid' or 'steady' belief that governs the 'forces of ... our actions'.
An idea assented to feels different from a fictitious idea that the imagination alone presents to us; and I try to explain this difference of feeling by calling it ‘a superior force’, or ‘liveliness’, or ‘solidity’, or ‘firmness’, or ‘steadiness’. This variety of terms, which may seem so unphilosophical, is intended only to express the act of the mind that makes realities more present to us than fictions, causes them to weigh more in thought, and gives them a superior influence on the passions and the imagination. Provided we agree about the thing, we needn’t argue about the labels. . . . I admit that it is impossible to explain perfectly this feeling or manner of conception ·that marks off belief·. We can use words that express something near it. But its true and proper name is ‘belief’, which is a term that everyone sufficiently understands in common life. And in philosophy we can go no further than to say that it is something felt by the mind which distinguishes the ideas of the judgment from the fictions of the imagination. It gives them more force and influence, makes them appear of greater importance, anchors them in the mind, and makes them the governing forces of all our actions.
This definition will also be found to fit perfectly with everyone’s feeling and experience. Nothing is more obvious than that the ideas to which we assent are more strong, firm, and vivid, than the loose dreams of a castle- builder. If one person sits down to read a book as a romance, and another reads the same book as a true history, they plainly receive the same ideas in the same order; and they attach the very same sense to what their author writes, despite the incredulity of one and the belief of the other. His words produce the same ideas in both, but his testimony doesn’t have the same influence on them. The believing reader has a livelier conception of all the incidents. He enters deeper into the concerns of the persons; he represents to himself their actions and characters, their friendships and enmities; he even goes so far as to form a notion of their features and manners. While the disbelieving reader, who gives no credit to the testimony of the author, has a more faint and languid conception of all these particulars, and can’t be much entertained by it unless he is held by the style and ingenuity of the composition. (Treatise, I.iii.7.)