(1) How do we know that anything continues to exist when we aren't perceiving it?
(2) How do we know that there is anything external to us?
(3) How do we know that there is anything that doesn't depend on our minds?
In the previous post we looked at Shepherd's answer to the first question, and it turned out to be fairly straightforward. Externality is a bit more tricky than continuity. In one sense, however, her answer here is the same as it was to the first question: Given that what begins to exist has a cause, the externality of the external world is known by reason.
What we clearly need in order to answer this question is some line of demarcation between the internal world and the external world. Shepherd argues that we have such a line of demarcation in sensation itself:
Inward existence is the capacity for sensation in general; outward existence is the exciting cause for some sensation in particular. The one is the very mind itself, or the power of thought and feeling; the other is a motive, or cause for a particular kind of it, and therefore out of, and distinct from, the continually existing essence of it. That is inward existence, of which the individual only is conscious; that is outward, which is in relation to the organs of sense, and to motion, in order to be apprehended, and must be met by them before it becomes inward; and which is so situated as to meet the organs of sense, and reply to the motion of others, (others being supposed possible,) as well as our own. (EPEU 40-41)
It's easy to miss here, but the key point is that the line between inward and outward is drawn by the sense organs themselves. Shepherd sees causal interaction between two objects as a kind of 'mixing' of properties. If I put my foot in clay, some of the properties of my foot 'mix' with the properties of the clay so that the resulting mixture, the footprint formed around my foot in the clay, is the necessary effect of the cause -- for the effect to be any different would require that there be properties of the foot or of the clay that were different. This 'mixture' account of causal interaction applies quite well to the case of sensation. In sensation we have two objects, ourselves and the world around us, and sensation is the two mixing together. The appropriateness of talking about 'mixture' in this case is overwhelmingly obvious in the case of smell and taste; but it applies also to hearing, touch, and sight. What Shepherd recognizes is that this means that sensation can only be understood in terms of our use of our sense of organs. This might seem a bit obvious, but it's important to understand that this means that classical empiricism, the empiricism of Locke or Berkeley or Hume, is false. Classical empiricism takes sensible qualities as primitives: colors, sounds, and so forth, and any consideration of sense organs themselves is usually an afterthought. If Shepherd is right, however, even our most basic comprehension of sensation involves causal reasoning about our sense of organs. Empirical life is shot through with causal reasoning from the beginning.
To get an idea of how this works, we first need to remember that Shepherd's basic framework for answering the question about externality recognizes two distinct causal factors. First, there is the object that is the self or mind, understood as the continuing capacity for sensation in general. This capacity is for sensation in general because we know that we can have many different kinds of sensations, both at once and in succession. But in order for us actually to sense anything, causes in the world have to 'mix' with this capacity for sensation in general. This brings us to the second causal factor in sensation, which are causes of particular sensations. These are precisely the same causes that we recognized before as continuing to exist. For our mind, as the capacity for sensation in general, to sense anything, some particular difference has to be introduced into it. The causes in the world do this, and the introduced difference is actual sensation itself. From this it directly follows that the things that cause us to have sensations are outside the mind.
This is not quite enough to give us a complete account of externality, however, because when we say that something is outside the mind, we mean more than just that it is not in the mind; we mean that there is a world outside the mind that surrounds us, that we move through it, and so forth. That is to say, when we speak of the outwardness of objects, we don't usually confine this to mere outwardness, but attribute particular kinds of characteristics to that outwardness. The externality of the external world is not merely a non-inwardness; we regard ourselves as in some way actually experiencing that outwardness. This leads Shepherd to identify several kinds of experiences from which we, by causal reasoning, get our idea of outwardness.
(1) The relation of objects to the sense organs. Among the objects that we recognize as having both continuity and externality in the sense we have just indicated are the sense organs themselves, to the extent that we sense them. What we experience with regard to other external objects is the use of our senses as instruments for gathering the effects (sensations) those other external objects cause. Thus we think of objects as external when we use our sense organs to get sensations of them:
Now the mind always referring the sensible action of any sense, to the mechanical action of its respective organ, (as an effect to its cause), and considering this mechanical action as existing in relation to those other objects, or causes, which are likewise needful to introduce the ideas of sensible qualities into the mind, does thereby truly perceive and detect the presence of such other objects as are external to, and independant of mind in general. (EEPU 57)
This would be why, for instance, that we tend to treat 'external world' and 'sensible world' as synonymous: much of our experience of the world as external comes precisely from the fact that this experience involves a constant use of sense organs, which are themselves outside the mind. It is also this point that will begin Shepherd's account of how to deal with the apparent sensations of dreaming: the sensations of dreaming don't involves any clear experience of sense-organ-use, and certainly not in the sense in which they are used in waking life.
(2) The relation of the sense of self to the sense of extension. Our experience of ourselves as continuing selves doesn't involve any clear sense of ourselves as spatial in character, but there is a sense in which this nonspatial self is located in space. That is to say, we don't experience our capacity of sensation as itself having any spatial characteristics. But we do have the experience of moving around in space, so we can take ourselves as being a capacity to move around, as well as (or perhaps as part of) a capacity for sensation in general. In order for us to have this capacity, however, there must be a distinction between rest and movement, and the latter has to have a close connection to distance. The relation to distance is a sort of possibility of movement with respect to distance, and this possibility of movement is part of our experience of the externality of the world.
(3) The relation of the world to our skin. The skin, understood as that which we experience as the boundary of the sense of the body's own extension, forms a sort of limit to our conscious capable to feel pleasure, pain, and the like, and therefore in itself it serves as an organ demarcating a line between inwardness and outwardness that affects how we understand the externality of the external world. All that we are is in some sense within our skin, and we move our skin about in the world in order to experience that world. What is at the skin, and, in an odd way, even some of what is in the skin, has a sort of immediate tangibility, but other things have only a possible tangibility to the extent that they come into contact with the skin. This mere possibility of tangibility is a key part of our sense that things are 'outside' us. The mere fact that we experience ourselves as having a skin at all is proof on its own that there really are objects external to us.
(4) The sense of the medium. We can add to these three a fourth experience, this one pertaining to sight. By sight we in some sense see the outwardness of things, see that they are external. The world comes in different colors, and those colors in relation to each other give a sense of extension, and the different parts of the world that give us a sense of coloring have to be so related to each other that they readily appear according in the various forms and relations we see ("proportional positions," to use the phrase Shepherd uses). Taking our bodies, within our skins, as a reference point or center, we find that this visible extension is intimately related in our experience to other qualities, like tangible distance, hardness, and so forth. Thus the causes of these qualities in the world must have a proportional relation to each other making it so that they readily give these sensations at irregular calls of the senses, and thus all of our sensations are, so to speak, bundled in with this visible sensation of a medium in which different colors and shapes have a relation to ourselves as a reference point.
All four of these phenomena affect how we conceive of the externality of the world, then, but all four involve causal reasoning of some kind or other, by which we take these experiences to have causes that by their nature fall outside our capacity for sensation in general, which is to say, outside the mind.
Thus here, as elsewhere, Shepherd is able to use her account of reasoning, much stronger than Hume's or Berkeley's, to argue for the rationality of the common-sense approach to the external world. If you challenged an ordinary person to prove that there was anything outside the mind, they might well start by trying to argue that they see and touch and move through the external world, and that things keep existing when they are not 'internal' to us by sensation. Our sense of the world as outside our minds is very much a sense of ourselves and the world interacting together as cause and effect, in both directions. This is, on Shepherd's account, all good, rigorous reasoning, and while Shepherd thinks most people only use very simple forms of this reasoning, she will insist firmly that the reasoning even in these simple forms is quite good. There is, however, one complication that I have somewhat glossed over in discussing this, and it is the fact that Shepherd thinks that the common-sense view of the world does tend to get one thing wrong when it comes to the externality of the objects we sense. We assign too much to external objects.
Except for Berkeley as the most notable exception, almost all early modern philosophers hold that in ordinary life we constantly make the mistake of externalize mental qualities. In particular, most of the sensible qualities we experience, like colors, are purely mental; they do not exist in the world. We make a mistake in treating the colors we see as properties of the world rather than properties of our minds. Berkeley is the exception because he thinks that the ordinary person is quite right here: there is no external world, on Berkeley's view, except the world that we sense. On his view the world just is the sensory ideas in our mind. Shepherd rejects this and accepts the usual claim that we mistakenly externalize our sensations. Interestingly and ironically, she does so because of a position that she shares with Berkeley. One of Berkeley's arguments against there being any other external world beyond our sensible ideas is that a sensation or idea cannot be like anything else except a sensation or idea; from this he concludes that there is no way to know anything about any alleged external world beyond our sensations. Somewhat strikingly, Shepherd agrees with the premise, but on her account of causation we have proven that there are things in the world beyond our sensations, and thus we know by causal inference that there is another world besides the sensations in our minds. Given the way she argues there is some similarity between the external world and our sensations, but it is at a very abstract level: the causes in the world have to be related in some way such that their effects have such-and-such relations and proportions. If I go outside and see the green on the leaves, I can know that there is something in the world that is related to other causes so that this leaf looks green and that leaf looks green, and so forth. But I cannot say what it is in the world that makes the leaves look green, beyond the fact that the things in the world have something that is cause-of-green-appearance. When we go about our business in the world, however, we regularly assume that the sensible qualities appearing to us are in the world itself -- that the leaves really do have this green color in and of themselves. Shepherd, like most early modern philosophers, denies that we can know this.
While she tends to support the rationality of the ordinary person's approach to the world, then, she does think that we go too far. She is not a direct realist but an indirect realist. The sensible qualities we experience are purely effects of causes that are known only by those effects; they are not actually images of those causes -- at least, we do not know that they are. But because of this way of looking at things, she also, unlike many of her contemporaries, can have a very simple account not only of why this is a mistake but also of why we make it and why it's a perfectly reasonable mistake, a mere inadvertence rather than a sign of some deep irrationality in human nature. Because the causes in the world are known only through their sensory effects, we use the sensory effects algebraically (as she puts it): in order to talk or think about what it is in the world that causes this green appearance, I use this green appearance to stand for its cause. Just as x in algebra stands for whatever quantity might be relevant, so too green in our thought stands for whatever it is that makes things look green. Our sensible experiences are signs of the world, and we use these signs to represent their causes and the relations between those causes. Because of that, all our words covering sensation can mean three different things: the sensory appearance in our minds, which is the effect; the otherwise-unknown cause that mixes with our sensory systems in order to have this effect; and the whole complex of the cause-causing-the-effect. This ends up being the core of much of her criticism of Berkeley, whom she thinks regularly equivocates among these three different meanings. But we can see why it's easy to make the mistake: knowing the causes only through the effects, we have to use the effects to stand for themselves, for their causes, and for the whole action of the causes-causing-the-effect, which means that we have to be thinking quite analytically in order to recognize that these are actually distinguishable things. In practical life there's no particular reason to think that analytically all the time; but she has sharp words for philosophers who are supposed think analytically but still engage in such equivocations.
That takes account of the outwardness of the world; now we have to consider independence.