For if one is unable to know anything about the external world, then one can not make any claims about it at all – even claiming that knowledge about it is impossible, because that too is knowing something about the external world – namely, that it is unknowable.
In fact, wouldn’t you need to bypass your own perceptions and go outside your own mind in order to make such a claim? After all, according to the argument, your own perceptions and mind are unable to determine anything about the external world. Given that argument, you would need to employ some means – other than your own perceptions and mind – to be able to verify whether or not an external world can be accessed by your internal perceptions and mind.
Because isn’t it possible that our perceptions are a dependable means of obtaining knowledge of the external world?
If we are to know anything, then don’t we need to (somehow) have access to that object of knowledge? And to have access, don’t we need a means by which we access it?...
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.... Aren’t sensory perceptions the means by which we gain access to – and knowledge about – the external world? Skeptics misrepresent their critics as identifying perception with the world itself. Rather, aren’t skeptics the ones conflating process with result; confusing the road with the destination; and identifying addition, subtraction, multiplying and dividing with the solutions of algebraic problems?
And one final observation: It seems to me that the skeptic is rigging the game from the start – taking away the means by which we can have knowledge of the external world in order to prove it is impossible to know anything about it. Which actually reveals another logical issue – that of assuming what is to be proven and then “proving” it (the fallacy of begging the question):
The skeptic assumes and asserts that we do not have the means by which we can have knowledge of the external world and, therefore, we can not have knowledge of the external world... — Thales
The issue is in the definition the "external world", as you point out. External is usually taken to mean, something outside our minds. So how it that we can go outside our minds (or perceptions that arise in persons with minds) to a completely external world?
In theory I suppose, it would be nice to be able to go outside one's own body to compare if our perceptions are getting something right or wrong about the world as we perceive it. But of course, this is impossible, for a view outside ourselves - and hence outside a framework of understanding - there would be nothing at all to experience.
The issue of correctness or incorrectness of our perceptions is not relevant about the external world, they are relevant in relation to our conceptions of our perceptions about the mind-dependent world: is that flower I am seeing white or grey? Is that the sound of a train or a concert?, etc.
A big issue, to my mind, is what exactly is meant by external here? People often speaking about external and internal, as if that distinction is very clear, I don't think it is. It would be replied that this sofa I am seeing is external to me, that is, it is not in my mind, so it is external in that sense.
But is that a substantial point? For the sofa I was seeing mere seconds ago tells me about how it looks to me, how it feels to me and how I conceive of and understand objects, always in relation to the being in question, in this case, a human being. So by this metric, the sofa I am seeing is not external to me, it is a representation, and representations are internal.
Perhaps a better distinction would be internal internal/ internal external, the former being ideas in my head absent an immediate object, and the latter would be objects as they appear to my senses and how my mind interprets them.
All this leaves aside the issue of the sciences, specifically physics, which is the star of the sciences, there we have good reason to believe that we are studying aspects of the mind-independent world, which is different from an "external world", because, at the end of the day, physics has to make sense to us in some manner, or it wouldn't matter at all, nothing would register.
It is in this area, in which we may come closest to something like the external world, but still with some caveats, which in fact make the science possible at all. Aspects of cosmology and classical physics seem to indicate some abstract properties of the mind independent world, which is fascinating, but structural, as in epistemic structural realism, which may be the best we can do. But, I could be wrong.
I think the knowledge you are discussing as veridical in hopes of showing that true perceptions tell us something about the external world, is more of an issue of accurateness of our perceptions given what many people report experiencing: if everybody sees the sky as blue, but I see it as yellow, then I may have some liver problems, or problems with my eye. But it doesn't reach the external world defined as, something outside our minds.