[lo_zamoyski]

How would you know? If all that's known is either known through the senses or drawn out by reason from what is known through the senses, then by declaring that sense data do not reflect reality, you've cut yourself off form the possibility of knowing reality altogether.

[lisper]

> How would you know?

Because that is the best explanation for what I observe.

> by declaring that sense data do not reflect reality, you've cut yourself off form the possibility of knowing reality altogether

That is true, but only in the uninteresting sense that I can never completely eliminate the possibility that I am living in the Matrix. So yes, it's possible that I'm wrong about the existence of objective reality. But if objective reality is itself an illusion, it's a sufficiently compelling illusion that I'm not going to go far wrong by acting as if it were real.

[lo_zamoyski]

> That is true, but only in the uninteresting sense that I can never completely eliminate the possibility that I am living in the Matrix. [...] But if objective reality is itself an illusion, it's a sufficiently compelling illusion that I'm not going to go far wrong by acting as if it were real.

That seems squishy, as what constitutes "going far wrong" is not meaningful under skeptical assumptions.

A better stance is one of cognitive optimism that avoids the irrationality of skepticism. Skepticism is irrational, because it leads to incoherence, and because there is no rational warrant to categorically doubt the senses. For doubt to be rational, there must be a reason for it. To doubt without reason is not to be rational, but to be willful, and willful beliefs cannot be reasoned with; they are not the product of evidence or inference — and they certainly aren't self-evident — but rather the product of arbitrary choice. The logical possibility of living in the Matrix is no reason for doubting the senses, just as the logical possibility of there being poison in your sandwich is no reason for doubting you'll survive eating it.

The difference between our positions is that I begin from a position of natural trust toward the senses and toward reason as the only rational possibility and default. I have no choice but to reason well or to reason poorly. I recognize that my senses and my inferences can err, but it does not follow that they always err. Indeed, the very claim that they can err presumes I can tell when they do.

So, if my inferences lead me to a position that undermines their own coherence, then I must conclude that my inferences are wrong (including those that led me to adopt a certain interpretation of, say, scientific measurements).

> Because that is the best explanation for what I observe.

But if your explanation involves contradiction of what you observe, then that is not only not the best explanation, but no explanation at all! An explanation cannot deny the thing it seeks to explain. Thus, by denying the objective reality of what you perceive, you are barred from inferring that denial.

[lisper]

> what constitutes "going far wrong" is not meaningful under skeptical assumptions.

I can be more precise about this. It means that the predictions I make on the basis of this assumption are very likely to be correct.

> Skepticism is irrational

No, it isn't. The vast majority of my beliefs about the world are not a result of direct observations, but nth-hand accounts. I believe, for example, that the orbit of Mercury precesses, but not because I've ever measured it myself, but rather because I heard it from a source that I consider credible. But assessing the credibility of a source is hard and error-prone, especially nowadays. There is always the possibility that a source is mistaken or actively trying to deceive you. And even for things you observe first-hand there are all kinds of cognitive biases you have to take into account. So skepticism is warranted.

> I begin from a position of natural trust toward the senses

That will lead you astray because your senses are unreliable.

> if your explanation involves contradiction of what you observe

But it doesn't. At worst it involves a contradiction of what I think I observe.