objects vs features
[features picture]
First, what does knowledge of physical objects involve?
One way to approach this question is by contrasting objects with features.
Physical objects contrast with features in three ways
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\begin{enumerate}
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\item physical objects have boundaries whereas mere features do not. (This needs qualifying
because there is a sense in which we can regard features as having boundaries; but when it
comes to
features, the boundaries are merely projections. To see the contrast, consider that we could
all be permanently mistaken about the boundaries of a physical object but not about the
boundaries of a feature---another species could not discover a million years from now that
humans are wrong about the boundaries of this feature, that they thought it was one feature
whereas really it is two.)
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\item physical objects persist in a way that features do not. You cannot ask, concerning a
feature now, whether this is the same feature as some time ago. At least, you cannot ask this
in the same sense that you can ask it about a physical objects.
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\item physical objects can interact with each other in way that features cannot: they
can causally interact.
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\end{enumerate}
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Imagine not knowing anything at all about particular physical objects and living in a world
consisting entirely of features. Nothing interacts, there are only patterns.
And things outside your perceptual field do not exist.
From your point of view, the world is limited your perceptual field now.
[ducks picture]
The way objects are ordinarily arranged in space, so that one occludes parts of another,
prevents us from doing this in any simple way.
[features picture]
Recall my imaginary world of features. In this world there is no principled way of saying
where one object ends and another begins.
As I said, features differ from genuine objects in not allowing us to make sense of the question of
whether we are carving them at their joints.
So an ability to segment physical objects is not necessary for knowing anything about
mere features but it is probably necessary for having any knowledge
concerning particular physical objects.
When Hannah hides behind the logs and a girl later pops up, we can ask whether it is Hannah
again or another girl.
That is, we know that objects can persist despite disappering from view---and despite becoming
entirely imperceptible.
[features picture]
Contrast features again. You might see this red feature moving across the scene.
But suppose it disappears and then, later a similar looking feature appears.
There is no fact of the matter about whether this is the same feature or a different one.
As I mentioned before, in the case of features we can't make sense of them as persisting over
time, or as there being interruptions in their presence.
I suppose, then, that to have knowledge concerning physical objects rather than merely
concerning features, it is necessary to be able to represent objects as persisting
even while unperceived.
Objects causally interact with each other; one pan supports another, two people collide and
bounce off each other. Relatedly, objects have counterfactual lives: sometimes you can say,
truly, that if that barrier had not been there, the car would be at the bottom of the valley
now.
[features picture]
As I mentioned, this is another respect in which objects are distinct from features.
Features do not causally interact with each other and they do not have counterfactual lives
either.
This point of contrast suggests that knowledge concerning physical objects as opposed to mere
features requires at least a limited ability to track causal interactions.