Knowledge presupposes a background of things held certain (that collection of animal and instinctive certainties – my certainty in my hands and feet – and very general claims such as that the world has existed for a long time) because that helps fixes the meaning of the claims about what I know or doubt.
In everyday cases, I cannot claim to know the background because I cannot claim to doubt it either and knowledge and doubt are symmetric.
Philosophical scepticism is, precisely, however, an attempt to cast doubt on the background.
But because the meaning of words on my lips is fixed by the background, any attempt to cast doubt on the background undermines the meaning of the very words I use to attempt to frame a sceptical doubt. (If ‘here is a hand’ might at best be used to teach someone the meaning of the word ‘hand’ then saying ‘Is this really a hand?’ – without the context of an industrial accident – will only show that the speaker does not know the meaning of the word ‘hand’.)
So one cannot put the sceptical doubt into words.
But in that case, it cannot be true (because there is nothing for the sceptical doubt to be).
This is akin to Putnam’s argument against ‘brain in a vat’ scepticism which goes like this.
There is a connection between the meaning of our words and what we can practically demonstrate. (I may be able to explain the meaning of some words using other words, but not all words. What alternative is there to practical demonstrations?)
If I am a person in the world, I can explain the meaning of the word ‘vat’ either by pointing to a vat or by explaining it using other words (like ‘metal’ and ‘bowl’) whose meaning I can – eventually – explain practically. So if I am a person in the world I can say / mean that I am a brain in a vat but – because I am a person in the world – it is false.
If I am a brain in a vat, I cannot give a practical demonstration of the word ‘vat’ which will fix its meaning as about real vats (because I am a brain in a vat and thus not in practical contact with the real world of trees and vats). So I cannot say that I am a brain in a real vat because my words cannot mean / reach out to that.
The best I could do would connect it to simulated / virtual reality vats. (I might ‘point’ to objects in my virtual reality with my virtual hands.) So if I am a brain in a vat, the best I could do is say that I might be a brain in a simulated / virtual reality vat (that is the best that my words would mean / reach out to). But that would be false (because I am not a brain in a simulated / virtual reality vat).
So I can never truly say that I am a brain in a vat. So that claim – that I am a brain in a vat – cannot be true. So brain in a vat scepticism is false.