Toddlers learn, often traumatically, that other people have different desires than they; but only my experience matters to me. A plum just now exploded in my mouth in extreme sweetness, delighting me and consuming all my attention, but if it could be miraculously reconstituted I would not recognise it among a dozen others. Often I learn more from forcing my own inchoate ideas into words than from listening to other people. Their perspective is too alien for me to take it in. Sometimes a hint from what you say may take root and germinate in me, coming to consciousness months or years later. I was embarrassed as well as flattered to hear from my friend that she could remember our conversations from years before: I cannot.
An ancient example of how our senses deceive us is that an arrow half in water appears to be bent, though it is straight. I can explain this from high school physics as the refraction of light in water. The arrow is straight: my senses deceive me in an explicable and predictable way.
Descartes said “je pense donc je suis” in considering how science works. He doubted that human beings could usefully learn and generalise from our environment. How can we know that our understanding does not fit reality simply by accident? He doubted that it could, and realised that those doubts came from somewhere, a self he called I. Cicero said “There is no statement so absurd that no philosopher will make it” but, rather, philosophers explore what we all take for granted, apparently ridiculously but eventually increasing understanding. Bertrand Russell wrote a book proving that 1+1=2.
If there is an external world, my understanding of it has to be constructed from my sense-impressions. So says David Hume. I did not understand this: of course the tree falling in a forest makes a noise even if there is no-one to hear it; but it depends what you mean by “tree”, “forest” and “noise”. The event is different from my experience of it and my understanding of it, so the event itself is unknowable.
Hume doubted the external world, then went out and played Backgammon with Edinburgh friends. He said it is human nature to believe in that world and interact in it, but Rationalist philosophy cannot prove it: we act on our programming, not on the World as it Is.
You might respond that this has nothing to do with human experience: we know things with varying levels of certainty, and correct our mistakes or drift deeper into delusion but rub along well enough. Philosophers themselves addressed that: Thomas Reid argued that philosophical theory must relate to the life you live in the real world. Centuries later, my understanding is enriched by both Hume and Reid, on how I can be wrong, and how I can be right enough.
Most of this comes from In Our Time. I have played a lot of In Our Time, mostly at night: before Melvyn Bragg says “With me to discuss” it has normally put me to sleep.