 Okay let's now move forward to Hume's theory of belief. Now to understand what's going on here let's recall the agenda. Back in 135 at the beginning of that he said we have three things to explain. The original impression, the transition to the idea of the connected cause or effect, and the nature and qualities of that idea. So he's just talked about the inference from the impression to the idea that's the second component. He's now moving on to the third one. So 137 of the nature of the idea or belief focuses on the idea that is the idea of the effect B which arises when we make an inference from the impression of the cause A. Well he said that all inference about the unobserved arises from causal inference and causal inference as we've seen moves from the impression of the cause to the idea of the effect. So we may establish as one part of the definition of an opinion or belief that is an idea related to or associated with a present impression. He's now going to build on that and see what else is required to make something a belief. He calls this a new question unthought of by philosophers that's from the abstract and it is actually quite a profound question and not an obvious one. Where in consists the difference betwixt incredulity and belief. And those of you who know about Phrygian logic may be interested to note that it's a similar question that Frege confronted and that he used the judgment stroke to solve. Suppose we have a proposition like P implies Q. Notice when you say P implies Q you're not asserting P you're not asserting Q. You're contemplating the proposition P and the proposition Q and a relation between them. So you can contemplate a proposition without asserting it without believing it. And what Hume is doing here is noticing that we can have a thought without actually believing it. So what is it makes the difference between a thought that we just ponder and a thought that we actually believe. It's quite a deep question. Well first of all it isn't the addition of some new idea. It's not that I have an idea of a particular thing and then I add to it the idea of existence and that makes it a belief because we know that there isn't any separable idea of existence. We dealt with that back in 1264. Another point if I believe proposition P and you don't you disagree with me the same ideas must be involved or it wouldn't be the same proposition. If I believe P and you disbelieve P if our ideas are different then we're actually at cross purposes. So the difference between believing and just pondering an idea a proposition must lie in the manner of conception rather than the content of the proposition. So it's not to do with the nature of the ideas themselves. It's the manner of their conception. And Hume talks about this as force and vivacity. The reason he goes for force and vivacity seems to be that it's the only thing he thinks the only kind of characteristic of an idea which could change the manner of the idea, the manner in which it appears to you without actually changing the idea itself. But it is quite problematic as we'll see. Okay so we can now fill out the definition of belief. An opinion therefore or belief may be most accurately defined a lively idea related to or associated with a present impression. So all belief in things that are unobserved according to Hume comes from causal inference. That means it has to start from what he loosely called an impression of the memory or senses. And it that leads us to an idea an associated idea which is a lively idea and that is what a belief is. Now I've said that appealing to force and vivacity is not entirely satisfactory. Suppose you're reading a novel. That might strike you with much more force and vivacity than the ideas that you get from reading a dull history. But you believe the history, you don't believe the novel. So how does that fit? If force and vivacity is what makes the difference to a belief, that doesn't seem to work. And how does force and vivacity fit into the theory of ideas? I mean Hume has said that all the perceptions of mind, the mind resolve themselves into impressions and ideas. Well is force and vivacity some kind of extra feeling? In which case presumably it's an impression. So why doesn't that make a difference to what you're thinking about? If the force and vivacity is part of the idea, if when you have a belief what happens is that the idea itself becomes more forceful and vivacious, then how can we distinguish between believing that there's a dull red door and imagining a bright red door? On the face of it the extra brightness there in our thought of the door provides more force and vivacity. So how does that not make it into a belief? Now Hume's talk about manner of conception seems to be rather happier. That seems to be more on the right track. Force and vivacity ought not to be something that goes within the ideas as it were, because otherwise we run into these problems very seriously. It seems more satisfactory to think of belief as some kind of manner of conception, the way in which we grasp or attitude to those ideas rather than the nature of the ideas themselves. But notice that that doesn't fit terribly well with Hume's theory of ideas. In particular it can provide some conflict with Hume's theory of personal identity. We'll come to that later. Hume wants to say that we don't have any conception of a person other than lots and lots of fleeting impressions and ideas, sort of bundle of perceptions, and it's not clear how a manner of conception can fit with that kind of picture. So there are quite a lot of problems here and we do see some symptoms of unease. So Hume added a paragraph to the appendix of 1740. The appendix came out at the end of 1740 along with book three of the treatise and he included in it a few extra paragraphs and footnotes and so forth to be inserted in books one and two. And this is one of them. An idea ascended to feels different from a fictitious idea. At this different feeling I endeavour to explain by calling it a superior force or vivacity or solidity or firmness or steadiness. It is impossible to explain perfectly this feeling or manner of conception. We may make use of words that express something near it but it's true and proper name is belief, which is a term that everyone sufficiently understands. So ultimately he seems to be rather giving up on explaining what belief is. He seems to be just saying, belief, you know, you know what that is, don't you? That's its name. And he hasn't really given us a very satisfactory account of what it is. However, back to 1739 and he does think he can give something like a satisfactory theory of belief. So notice that we've got two discoveries which have a rather neat connection. Causal reasoning starts from an impression of the senses or memory. They're distinguished from mere ideas by their force and vivacity. Indeed, that force or vivacity whom says there constitutes their belief or ascent. And now we've concluded that something that's inferred by causal inference becomes a belief in virtue of its force and vivacity. Well, that's a very nice coincidence. We get the hydraulic theory of belief, that's my term for it. I would willingly establish it as a general maxim in the science of human nature that when any impression becomes present to us, it not only transports the mind to such ideas as are related to it, but likewise communicates to them a share of its force and vivacity. I call this the hydraulic theory because it's as though you've got force and vivacity squirting into the inference from the impression. And then it goes into the idea, fills up the idea, and the idea becomes more forceful and vivacious. In fact, Hume goes further when he discusses probability, which we'll be coming back to in a future lecture. He extends this. You get the force and vivacity being divided into different channels. And that leads you to having different probable beliefs in the various possible outcomes. So he does seem to take the hydraulic model quite seriously. Okay, I'm going to end with two quite important passages from 138. Well known passages and quite significant. So he's summing up his theory of belief and he says, thus all probable reasoning is nothing but a species of sensation. It is not solely in poetry and music, we must follow our taste and sentiment, but likewise in philosophy. When I'm convinced of any principle, it is only an idea which strikes more strongly upon me. So it's as though we are simply creatures being buffeted around by these associational processes in our mind. They lead force and vivacity to be applied to certain ideas. When the force and vivacity is applied to those ideas, I end up with a belief. And belief is a sort of feeling. It's all about sensation. Very, very anti-rationalist notice. He's more or less saying that when we reason, probably, you know, probable reasoning, causal reasoning, essentially, it's the associational processes that are driving it, not reason. Again, it's looking pretty skeptical. Also quite importantly, the uniformity principle is typically unconscious. So when we infer one thing from another, causally, typically, we do it without a moment's thought. He gives the example of a man coming to a river and stopping at the river. And it's not that the man is reflecting on his past experience and thinking, well, if I go into the river, I could drown and so forth. The idea of the river and the idea of getting wet and drowning comes so immediately to him that he doesn't need to reflect on past experience at all. It's pretty much automatic.