It’s not obvious, in fact. It’s natural to read Plato as making a broadly logical point, rather than a psychological (or apparently psychological) one. The key points of the first passage here (437d-e) might be put like this:
The passage which most commentators take to be insisting that thirst itself is unlike any kind of value judgement would then be better understood as making these points:
This logical reading seems to be confirmed by what Socrates goes on to say about knowledge:
And a little later:
The reason why this seems to confirm the logical reading is that there really is no such thing as having knowledge without knowing anything in particular. Suppose you say, ‘I know’, and your friend asks, ‘What do you know?’ Could you say, ‘Oh, I don’t know anything in particular; I just know’? That would be absurd. So the point in this passage in the Republic has to be about different ways of specifying knowledge, not about different kinds of knowledge. The points which Socrates is makeing here about knowledge now seem to be these:
What would be the point of making the broadly logical point about thirst? To make sure that we end up with the right conflicts, between the right parts of the soul – parts which are broadly comparable to the three classes in the Republic’s city. What we want is a conflict between being thirsty (i.e., wanting a drink) and wanting not to drink at all. That’s because that will plausibly be a conflict between the appetitive part (which wants the drink) and the reasoning part (which is trying to control the body’s fluid intake). If we just had a conflict between wanting a hot drink, say, and wanting not to have a hot drink, that might easily be construed as a conflict between two different appetites, which isn’t what we need for the big analogy of the Republic.
The argument of the Republic IV passage turns on this Principle of Opposites:
If you take the broadly logical reading of the passages in 437-438, you’ll take this to be a logical principle – a form of the Principle of Non-contradiction. We might put it like this: Where being F and being G logically exclude each other, it’s impossible for the same thing to be both F and G in the same respect, and in relation to the same thing, at the same time. Now what happens in the main argument? We seem to have examples like this: The same person both wants a drink and wants not to drink at the same time. This seems to provide us with examples which have this form: The same person both wants a drink and does not want a drink at the same time.
But wanting a drink and not wanting a drink logically exclude each other. So if we do have examples like this, the Principle of Opposites says the same person must have the two wants in different respects—or, as we normally say, in different parts of the soul. If this is how the argument works, it looks as if Socrates here accepts that it is possible for the same person to have competing desires or contradictory beliefs at the same time—provided they’re in different parts of the soul. But this, of course, is something that Ordinary Jo denies. In fact it is only because Ordinary Jo thinks it’s impossible for the same person to have competing desires or contradictory beliefs at the same time that Ordinary Jo is committed to this key part of their view: If knowing what’s right and wrong were not a different kind of state from the wanting or desiring which actually affects your action, akrasia wouldn’t be possible.
So it looks as if we can make sense of the possibility of akrasia, without thinking of any wants as being radically different from thoughts about what’s right or wrong, good or bad – provided that we allow that the same person can have contradictory attitudes at the same time.