I take the position that the only thing knowable with certainty is current
directly experienced 'subjective experiencing' - mental experience. That's all
I can know is real, my current mental experience.
Everything else is inferred from that, with no 'bridge of certainty'. There is
no way of knowing for certain if my own body and brain exists, or anything else
'out there', or if my memories just popped into existence.
This knowledge isn't much use tho, because if I 'kick a rock' I have pain in my
'foot'. The only part of that scenario I can know to be true is the mental
experience, but if I treat it all as true (that rocks and my foot exists) then
there's no difference. So in reality I have to Act As If rocks and my feet
exist, in order to avoid the pain.
So Materialism works, and I Act As If it's True on the basis of Utility.
I create models of the external material world, with feet and rocks and other
people. And when I point to a green apple and ask if you see it too, you say
yes. The nature of subjective experience is innately Private, so I can't know
if you see what I see when you agree, but again, it works. And it working is
persuasive that it could mean it's real. As is the fact that the 'external
world' is incredibly vivid, it makes sense, I can understand its rules and make
testable predictions (by dropping the apple), it has a logic, it's cohesive.
I'm constantly tweaking and updating parts, but essentially it's a vastly
complex, cohesive model whose proof is in the pudding of it working. I call
this our Shared Reality for want of a catchier phrase. And Materialism also provides us within our Private Subjective experiencing with a Public lingua franca of Shared Knowing.
That doesn't mean the Shared Reality model is complete or accurate. The model itself tells me humans are fallible, designed on the basis of evolutionary
utility to navigate the world at a particular (useful) level of perceptual and
cognitive granularity. I can't directly see atoms or black holes because I
don't need to. Who knows what else about this external reality is hidden beyond our perceptual and cognitive reach? Nobody. We can makes guesses based on our shared rules of how the parts we see work, and sometimes work out ways to test them. But we don't know how vast our ignorance is.
It might be we have a pretty good scientific model for our universe which
includes everything that exists, with just a few holes. It might be there are
levels of granularity way beyond that. It might be there are other universes
way beyond ours. It might be things as fundamental as logic, maths, dimensions, matter, forces, time are just our own constructs, useful navigation tools, the maps not the territory. Even in our Shared Reality, we have to be humble about what we can say we know.
And then there's neuroscience and psychology. Brains are designed for utility.
Even on the most fundamental level of 'feeling' a table as solid, because
that's the useful level of granularity for navigating the world. And they are
extremely costly organs in several ways, so they evolve useful shortcuts, rough
n ready mechanisms which will usually do the job. So for example the whole
field of quantum mechanics feels counter-intuitive, irrational to us. We have
cognitive biases, we're fooled by illusions, our senses aren't like video
recorders, the images and sounds are constructed in our heads within an
incredibly complex feedback system linked with memory, emotion, etc. The
stories we tell ourselves to explain even our own actions and beliefs are
unlikely to be the full story.
But still we can learn more about our limitations, how to counter them, get
closer to truth in our Shared Reality, using tools like reason, empiricism, the
scientific method. Understand the world and ourselves better, as we realise how
little we know. That's a noble endeavour.