Frede maintains that Plato was the first thinker to confine the meaning of the Greek verb which is translated "to perceive" (aisthanesthai) to what we think of as "sense perception" and nothing else.
Plato did so to disentangle the farrago of perception, belief, thought, etc. that occurs in the Theaetetus from 151 onwards.
Here is the central argument which Frede wants to analyze:
The argument up to and including 187a purports to show that we should look for knowledge in what the mind does when it considers by itself questions about being, that is, when it forms beliefs.
Frede suggests that Plato thinks that we grasp the truth by means of forming beliefs that grasp the truth. All beliefs, Frede says, for Plato, are of the form 'A is F' (where A and F are anything you care to slot into a sentence of the form "A is F"). To believe that A is F is to attribute being to A and to F (and also to attribute F to A). Thus a true belief presupposes that one "has correctly settled questions concerning the being of something."
In 185 and 186, the mind considers both perceptual features of objects and intelligible features of objects. In the case of perceptual features, it draws on perceptual evidence (185b10-12) and something else (the mind/intelligence). In the case of intelligible features (such as being, sameness, difference, etc.), the mind draws on its own internal resources by itself, i.e. comparisons and reasonings (186a10ff., b8ff., c2ff.). In both cases, it is forming a belief: thus perceptual features and intelligible features are considered by the mind and both lead to beliefs.
To form a belief that A is F, the mind has to decide at least the following:
The perception itself is a purely passive affection (186c2, 186d2). Any belief requires an ACTIVE mind.
Since all that activity is needed to produce belief, and knowledge is a certain kind of belief, all that activity (at least) is required to produce knowledge.
So even the belief that "This is red" is NOT bare perception. Perception only gives us something like "redness": our mind must actively interpret that given "redness" into "this is red."
Plato does speak as though children can already from birth perceive many
things, and that might be taken to mean that Plato includes judgements IN
perception: in other words, perception might involve perceiving that "A is
F," and not simply being passively affected.
We might call that "labelling": one step beyond mere perception, it
involves labelling what one perceives and feels automatic "That is red"
doesn't require any reflection. Frede, however, reminds us that
later ancient philosophers, the stoics, insisted that children do not
perceive that A is F: perceiving that A is F involves a later stage in
development, which includes being able to articulate the thought, and to
form concepts, which infants cannot do. Only reflection on sense-data lead
to concepts and articulation. When we perceive, strictly speaking, we
perceive only the proper objects of each sense (color for sight, perhaps
shape, and something like size?).
Bailly: From the fact that later philosophers think that we do not form
beliefs right from birth, we cannot conclude that Plato thought that, but
if we have good reason to think that he should believe that, and no good
reason to think that he believed the contrary, that is a (weak) argument
that he did believe that.
"Belief is the result of a silent discussion one leads with oneself" (189e-190a).
Protagoreanism, however, assumes that beliefs are things that happen to
us via appearances: appearances simply give us beliefs, and those beliefs
are true. Frede says that Plato will not deny that there are some beliefs
that just happen to us (I am not sure what an example would be: perhaps
"perception" of a Form, or perhaps when we take someone else's beliefs and
just adopt them), but he certainly believes that sense perception does not
just give them to us.
Therefore, perception and knowledge are two different things according to
the arguments in Theaetetus up to 186.
Consider a device for turning on a light when a door is open and turning it off when the door is shut. The device has a sensor that detects whether the door is shut. If the door is shut, nothing happens. If the door is not shut, then the light is activated. The light stays on until the sensor detects that the door is shut. At that point, the light is turned off.
Does that device have perceptions? Does it have beliefs? The perception would be something like "shut door" and "not shut door", and the belief would be something like "The door is shut" and "The door is not shut." The way I have described it above uses the language of perception and belief, but is that language meant to be a precise description of what the device does, or does it rather treat the device as though it had perception and belief? Does the device judge that the door and shutness and openness exist or are? Does it grasp truth?
What if the "sensor" is just a button in the door frame like the button in a refrigerator door? The light goes on when the refrigerator door opens because the circuit is completed by opening the door, and the light shuts off when the door is shut because the circuit is interrupted by shutting the door. Does that constitute perception or belief in some minimalist way? If you think that is perception and belief, do you think that when a comet starts being pulled towards the earth as it gets closer that it perceives the earth, and that it believes the earth is closer?
The big questions here are whether there is anything different about us from these simple systems. If it turns out that we can chemically, electrically, and mechanically explain everything about ourselves like we can those simple systems, and so "perception" and "belief" turn out to be fully describable by means of chemistry, electricity, and mechanics, are we any different from "inanimate" matter in anything besides complexity? I am not sure we are, but when you add into the mix things like feedback loops, memory storage, projection programs, and self-assessment loops, you may add in something that is different from refrigerator door circuits and comets being attracted to the earth by gravity. I do not currently have an adequate way of explaining what that is, and it may be my own arrogance and bias towards animals, and more than that, humans, that is making me think we are different.