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The Obscurity of Sensory Perceptions

Ok, so I was saying this ...

How can we acquire knowledge about the essential nature of the bodies located outside us?

Sensory perceptions provide only very obscure information about the essential nature of bodies.

∴ Not by treating sensory perceptions as a basis for judgements about them.

How might you defend this claim? (That sensory perceptions provide only very obscure information about the essential nature of bodies.)
Recall this question

McCloskey et al, 1980 figure 1 (part)

McCloskey et al, 1980 figure 2D

\section{Impetus}

The person who sets the ball moving impresses in it a certain impetus, [which acts] in the direction toward which the mover was moving the body, either up or down, or laterally, or circularly’ (Buridan, 13xx; cited by McCloskey et al).

But what does this tell us about sensory perceptions?

People make wrong judgements. So what? What does this tell us about sensory perceptions?
\section{Perceiving Impetus}

representational momentum

Sometimes when adult humans observe a moving object that disappears, they will misremember the location of its disappearance in way that reflects its momentum; this effect is called \emph{representational momentum} \citep{freyd:1984_representational,hubbard:2010_rm}.
The trajectories implied by representational momentum reveal that the effect reflects impetus mechanics rather than Newtonian principles \citep{freyd:1994_representational,kozhevnikov:2001_impetus,hubbard:2001_representational,hubbard:2013_launching}. And these trajectories are independent of subjects' scientific knowledge \citep{freyd:1994_representational,kozhevnikov:2001_impetus}. Representational momentum therefore reflects judgement-independent expectations about objects’ movements which track momentum in accordance with a principle of impetus.% \footnote{ Note that momentum is only one of several factors which may influence mistakes about the location at which a moving object disappears \citep[p.\ 842]{hubbard:2005_representational}. %: %\begin{quote} %`The empirical evidence is clear that (1) displacement does not always correspond to predictions based on physical principles and (2) variables unrelated to physical principles (e.g., the presence of landmarks, target identity, or expectations regarding a change in target direction) can influence displacement.' % %... % %`information based on a naive understanding of physical principles or on subjective consequences of physical principles appears to be just one of many types of information that could potentially contribute to the displacement of any given target' %\end{quote} }

Hubbard 2005, figure 1a; redrawn from Freyd and Finke 1984, figure 1

Hubbard 2005, figure 1b; drawn from Freyd and Finke 1984, table 1

\textbf{Representational momentum suggests that there are automatic processes which predict the future trajectories of physical objects.}
You can do RM with the spiral setup too ...

Freyd & Jones, 1994 figure 2

predicted memory shifts: the representational momentum effect you would get if your subjects’ representational momentum were predicting the straight path.

Yet ‘our subjects had relatively accurate conscious knowledge of the trajectory of a ball exiting a spiral tube (63% to 83% chose the correct path; only 4% chose the spiral path).’

\citep[p.~975]{freyd:1994_representational}
it really is perception!

‘subjects showed a memory shift for a path that the majority of subjects did not consciously consider correct’

\citep[p.~975]{freyd:1994_representational}

‘the representational momentum memory shift for a ball following a spiral path after exiting a tube is greater than the memory shift for a ball following the physically correct linear path. A curvilinear path, midway between the spiral and straight paths, produces shifts midway between those for the other two paths’

\citep[p.~975]{freyd:1994_representational}

Yet ‘our subjects had relatively accurate conscious knowledge of the trajectory of a ball exiting a spiral tube (63% to 83% chose the correct path; only 4% chose the spiral path).’

\citep[p.~975]{freyd:1994_representational}

‘subjects showed a memory shift for a path that the majority of subjects did not consciously consider correct’

\citep[p.~975]{freyd:1994_representational}

Freyd & Jones, 1994 p. 975

✓

How can we acquire knowledge about the essential nature of the bodies located outside us?

Sensory perceptions provide only very obscure information about the essential nature of bodies.

∴ Not by treating sensory perceptions as a basis for judgements about them.

This is all by way of a demonstration that Descartes was right. Sensory perceptions really do provide only very obscure information about the essential nature of bodies.
Just one small problem: how are we going to answer this question?

one last thing

Bennett mentions something Turnbull says about Aristotle's science:

Aristotelian physics ‘is reasonably effective for organizing bodies of knowledge.

From the perspective of modern physical and biological science, however, it is severely crippled by its close linkage with what Wilfrid Sellars calls ‘the manifest image’, i.e. what is available to us by means of our very limited sense organs. [...]

The tie to entities known through perception prevents access to—much less the discoveries of—modern physics (and, consequently, chemistry and biology)’

(Turnbull 1988, p. 120 cited by \citealp{bennett:2003_learning}.)

Turnbull 1988, p. 120

This is what Descartes saw, and that’s what he is trying to explain in the Meditations.
The usefulness of extensive doubt “lies in freeing us from our preconceived opinions, and providing the easiest route by which the mind may be led away from the senses.”
In studying the Mediations, we can have a lot of fun with sceptical arguments involving dreams and demons, and other bits and pieces. These have been widely examined by philosophers, most quiet critical. What you must never forget is that Descartes’ fundamental insight was correct--- Adherence to Aristotelian physics is based on the assumption that the model of the physical implicit in human sensory perception is the truest, most accurate model of the physical there is; and this assumption is incorrect.