If all of which I am directly aware are my own thoughts and perceptions, how can I be certain that there is a physical reality that exists independently of me?
Answer by Craig Skinner
Your question states nicely what is called philosophical scepticism about the external world. The short answer is that you can’t be certain that there is a stand-alone physical reality.
Scepticism, as a philosophical method, was famously used by Descartes, notably in his Meditations. He begins by doubting everything except one thing that can’t be doubted, namely,
* because he is doubting, he is thinking, and so must exist (‘I think therefore I am’, in Latin ‘Cogito ergo sum’).
He hopes to argue his way back to most of his former beliefs by sound reasoning from this single, clear and distinct, indubitable belief, thereby establishing a ‘firm and permanent structure in the sciences’.
First, he doubts everything learned through the senses (empirical or a posteriori knowledge as we would say). For the senses can deceive, and also, at any given moment, he can’t be certain he is not dreaming, or that his mind is not controlled by a ‘evil genius’ which deceives him about everything.
Secondly, he doubts all truths of reason (rational or a priori knowledge). He feels that, even if dreaming he knows that 2+3 = 5, but considers that an evil genius could deceive him even about mathematical truths, interfering in his thought every time he adds 2 and 3 so that he is sure (wrongly) that the sum is 5.
His arguments back from the cogito to belief in the physical world of concrete things, other people and his own body, are widely considered unsound or invalid. The upshot is that one of his main legacies is scepticism rather than its resolution, and strong philosophical scepticism (about matter, the external world, causation and selves) later emerges, for example in the views of Berkeley and Hume.
To return now to your question:
Things clearly seem or appear a certain way to you (you speak of ‘ my own… perceptions’)
So the philosophical (metaphysical) question is ‘what accounts for these appearances?’
The possibilities include:
1. Everything (universe and contents including other people and your own body) is just ideas in your mind which is all that exists (Solipsism).
2. An external world exists, but is made of ideas. All that exists is ideas and minds, including God’s and those of other finite beings such as you and me (Subjective Idealism, as advocated by Berkeley).
3. An external world exists, and you are in it, but not experiencing it as you imagine. You are a body in a pod (as in the Matrix) or a brain in a vat, in the real world but utterly unaware of it, being stimulated by mad scientists to experience things just as if they were really happening to you.
4. An external world exists, but you are not in it. You and everybody/everything else constitute a simulated world. You are a virtual being. This assumes that very sophisticated computer programs could produce virtual beings that are conscious (I don’t see why not). The simulation of which you are a part might be being run by, say, 26th century humans, or by superior, non-human intelligences.
5. There is a stand-alone physical reality.
I’ve never had any enthusiasm for (1), I don’t believe in gods, so (2) is out, I can’t imagine why anybody would want to systematically deceive a brain for years, and so discount (3). That I am a simulation (4) is a distinct possibility. Of course those doing the simulation are in the real world. But there is only one real world, whereas there could be vast numbers of simulations running in that world. Hence, statistically, any given person, say you or me, is more likely to be in a simulation than in the real world. Still, all we can do is try to lead our virtual lives so that the simulation controller stays interested and doesn’t delete us or switch the whole thing off.
On the whole, I go for (5). Of course matter gets less material with every advance in physics. Once, it seemed substantial, composed of many tiny, hard, atoms. Now, it seems, ultimate things (if indeed ultimate) such as electrons, quarks, photons, are reduced to vibrations of immaterial fields, or to information. And, none of these fundamental constituents of matter seems to have any size at all, which of course explains how everything in the universe could have been present in a point-like singularity before the Big Bang. Maybe the real world is just as much a computer program as any simulation, except it’s one with no designer which runs itself.
These sceptical thoughts serve to remind us how little (if anything) we can know for certain. But we manage to live just fine without certainty. Descartes didn’t really doubt that there is a a world out there, or that he had a body. His scepticism was a ploy (methodological scepticism) to try to put his views on a rational footing. And, while Hume encourages us to doubt that we can KNOW there is an external world, or a connection between cause and effect, or a connection (a self) uniting all those thoughts and perceptions of yours (he was an epistemological sceptic), my reading of him (not everybody’s) is that he never really doubted the EXISTENCE of an external world, a cause-effect connection, or selves (he was not a metaphysical sceptic).