I've been going back to a piece I'm trying to write about character and consistency in the Philebus. Along the way, I've been wondering about what you might call Protagorean hedonism, namely the idea that all pleasures are true – true in the same sense that Socrates wants to argue that some pleasures and false – and from there back to the Theaetetus. At Theaetetus 178d8–e6, Socrates argues that a cook is a better predictor than a dinner guest of whether a meal will be pleasant:
Or suppose a dinner is being prepared. Even the guest who is going to eat it, if he has no knowledge of cooking, will not be able to pronounce so authoritative a verdict as the professional cook on how nice it is going to be (περὶ τῆς ἐσομένης ἡδονῆς). I say 'going to be', because we had better not at this stage press our point as regards what is now pleasant to any individual, or what has been in the past. Our question for the moment is, whether the individual himself is the best judge, for himself, of what is going to seem and be for him in the future.
Trans. M. J. Levett rev. M. F. Burnyeat
We might find this peculiar since, after all, de gustibus nil disputandum. I am not inclined to think that if I dislike a meal at a restaurant I should defer to the chef's opinion that it is in fact delicious. But Socrates is clearly less reticent in affirming that matters of gastronomic pleasure are analogous to matters of health, for example, in being the province of a kind of expertise. An uneducated palate might well not take pleasure in something it should. The other important point is that Socrates is not considering cases in which a chef will tell a diner that what he thinks is not pleasure is in fact pleasant. Rather, the point is about predicting what will be pleasant. Here perhaps this is not such a peculiar thought. After all, famous chefs regularly produce dishes which an inexpert diner will predict are not pleasant (e.g. snail porridge) but which are in fact very nice. The chef's command of taste combinations allows him to produce surprisingly delicious new dishes. Still, we might reasonably insist that not all diners are the same and what one persons may enjoy another will not. So chefs might need to know something about their clientele and personal preferences.
All the same Socrates' cautious restriction to talk about future pleasure does not show that he is at all committed to the thought that when it comes to the estimation of present pleasure each person is an authoritative and incorrigible guide. Not only is it clear from other discussions that Socrates has a tendency to think that people are often badly mistaken about their current state of pleasure or pain, but it is evident that the restriction to future pleasure in this case is merely for argumentative convenience at this point in his exploration of Protagoras' view. Even granted that restriction, Socrates and Theaetetus are inclined to think that an individual is not necessarily the best judge of what will be pleasant to him. Insofar as such a person can be mistaken (e.g. by believing on Monday that he will not like snail porridge but finding it pleasant on Tuesday), then this will offer a useful example of a false belief to add to the mounting case against Protagoras' assertion that 'main is the measure'.
It seems to me that the Philebus adds to this analysis the idea that rather than a belief on Monday that such-and-such will or will not be pleasant, we might instead talk about a pleasure or pain on Monday in anticipation of a supposed pleasure or pain on Tuesday. If I am pained on Monday by the imagined experience of eating snail porridge on Tuesday but then enjoy the snail porridge on Tuesday, then Monday's was a false pain.