Skepticism about the senses

Class notes for 25 January

Main points

I began by finishing what I had to say about necessary connection.

Then I gave an overview of part four, “Of the sceptical and other systems of philosophy.”

The bulk of the hour was spent going over 1.4.2. I criticized Hume’s explanation of the vulgar belief in the continued existence of objects while unperceived.

But my main aim was to show how the argument in this section is a skeptical one, that is, how it is supposed to show that we have no reasons to believe what our senses seem to tell us, namely, that there are external objects. (A skeptical argument is different from one that attempts to show that some belief is false. The attack on the vulgar system is not a skeptical one, for example, since it holds that there is a clear thing that we should believe, namely, that the vulgar system is false. A skeptical argument seeks to show that we have no reason to believe something, not that we have clear reasons not to believe it.)

Necessary connection and projection

I illustrated the philosophical and natural definitions of causation by comparing them with other relations (see I said that what was distinctive about our idea of the causal relation, the natural definition, is that it includes the idea of necessary connection between the cause and the effect.

The impression, and thus the idea, of necessary connection are feelings that cannot represent anything (see; in this way, they are unlike the impressions and ideas of other relations which do represent something, namely, the relations.

So why do we think that causally related objects (or causally related perceptions, for that matter) have a necessary connection to one another? Hume’s answer is that we project the feeling onto the objects (see I said that I did not see how this made any sense and that I thought that Hume was trying to have it both ways.

Hume’s reason for saying that the idea of necessary connection comes from a feeling is that we can’t imagine how objects could be connected in this way. Feelings don’t run afoul of that because they don’t represent anything. My feeling a tickle doesn’t represent anything, it’s just a tickle. By contrast, my idea of the relation of spatial distance does represent something: the distance between two things.

But if that is so, I don’t understand what is involved in projecting feelings onto objects: that strikes my ear as taking back everything in the previous paragraph. If it does not do that, how does our projection involve a coherent thought about the causally related objects?

What do the vulgar believe?

The vulgar system does not distinguish between perceptions and objects; the philosophical system, by contrast, does.

So when we have our vulgar beliefs, what is it that we believe? Specifically, how do we vulgarly regard our perceptions?

One very curious thing that Hume says is that we regard them as perceptions. Here are the relevant passages.

If our senses, therefore, suggest any idea of distinct existences, they must convey the impressions as those very existences, by a kind of fallacy and illusion. Upon this head we may observe that all sensations are felt by the mind, such as they really are, and that, when we doubt whether they present themselves as distinct objects, or as mere impressions, the difficulty is not concerning their nature, but concerning their relations and situation. Now, if the senses presented our impressions as external to, and independent of ourselves, both the objects and ourselves must be obvious to our senses, otherwise they could not be compared by these faculties. The difficulty then is, how far we are ourselves the objects of our senses. (

[impressions] appear, all of them, in their true colours, as impressions or perceptions. And indeed, if we consider the matter aright, it is scarce possible it should be otherwise; nor is it conceivable that our senses should be more capable of deceiving us in the situation and relations, than in the nature of our impressions. For since all actions and sensations of the mind are known to us by consciousness, they must necessarily appear in every particular what they are, and be what they appear. Every thing that enters the mind, being in reality as the perception, it is impossible any thing should to feeling appear different. This were to suppose, that even where we are most intimately conscious, we might be mistaken. (

… all impressions are internal and perishing existences, and appear as such … . (

This sparked some good questions at the end which may be summarized as: if we don’t distinguish between perceptions and objects in our vulgar beliefs, what does it mean to say that, in our vulgar beliefs, we take our perceptions to be perceptions?

One thing that it can’t mean is that we take our perceptions to be distinct from objects: as Aidan pointed out, that is precisely what the vulgar belief does not involve. (I think I slipped on this point in the middle of the lecture: apologies.)

But “our perceptions appear as perceptions” has to mean something. The problem that Hume sets out to answer is why we would have the idea of the continued and distinct existence of objects given that we only have perceptions. There is something about the fact that we only have perceptions available to us that makes the belief in the continued and distinct existence of objects a problem, something that calls for a verrrry long explanation.

I had taken him to mean that we vulgarly take perceptions to have all of the qualities that the philosophers attribute to perceptions except the contrast with objects. Hume believed that the philosophical system is correct in holding that our thoughts and feelings are all just perceptions. Since that’s what they are, they have to have the properties of perceptions.

At least one feature that he clearly singled out is the fact that, “… all impressions are internal and perishing existences, and appear as such …” ( “Internal” is a problem: see Aidan’s remark above. But “perishing” isn’t. The problem, as Hume set it up, is that we have perishing impressions but believe we perceive non-perishing things: objects that continue in existence when not perceived and whose existence is distinct from the mind.


Will suggested an alternative interpretation that several others said they shared. It is that what Hume was saying is that we are certain about our perceptions. Since time was short, I didn’t get a chance to fully understand the point. Perhaps these remarks will be helpful.

First, it seems to me that Hume said we are certain that our perceptions are perceptions.

Second, we may also be certain about the content of our perceptions, that is, we may be certain of what they are perceptions of. So, I may be certain that this is a perception of a cactus or that is a perception of a cat. However, I don’t see that as being what is at issue here, that is, I don’t see that Hume either asserted or denied it.

One consequence of his description of the vulgar belief is that we do not typically have a particular kind of doubt, namely, that the way things are could be radically different from the way we perceive it to be. This is so because we do not vulgarly distinguish between the way we perceive the world to be and the way the world is. But not having a particular kind of doubt is not the same thing as certainty: I could have other doubts about what I believe.

We shouldn’t take that too far, of course. Even in my good old everyday vulgar mode, I’m perfectly aware that I may be wrong about things. “It looks like a crow, but it may be a raven.” “Why did you bring me an avocado, I asked for the tickets to the Mikado?”

What’s wrong with Hume’s explanation?

In a nutshell, Hume imported what he was trying to explain.

He was trying to explain the belief in the continued existence of objects even when they are not perceived.

His explanation is that the imagination cooks up this idea as a fiction to resolve a conflict between two other things that we believe. So we have three things.

  1. My belief that resembling perceptions that I have at different times are perceptions of the same, numerically identical thing.
  2. The obvious fact that the resembling perceptions are different.
  3. The fiction of a continuously existing thing.

Here’s the story. 1 and 2 are in conflict with one another. The imagination creates 3 to resolve the conflict between 1 and 2. 3 is the idea of a continued existence. The story about the conflict between 1 and 2 and its resolution is the explanation of how we get the idea of continued existence.

But, I said, 1 is the idea that Hume set out to explain: it is the idea of a thing that exists independent of my perceiving it. If I did not think that, there would not have been any reason to think that my perceptions were identical, rather than being numerically distinct but qualitatively identical.

In other words, if I had not already had the idea of objects that exist without my perceiving them, I would have thought that things blink in and out of existence when I perceive them … and conveniently so! I would not have had any reason to think that the different perceptions were identical in any sense if I did not think that they are perceptions of something that exists apart from my perceiving it.

Since the belief in 1 assumes that I believe in the continued, independent existence of objects, it cannot explain why I believe in the continued, independent existence of objects.