The Solipsist's Fallacy
Solipsism is the belief that you can never truly know if anyone else is real.
This is a common endpoint for philosophy, the reductio ad absurdum at the end of long chains of reasoning. It's what Descartes was trying to get around when he wrote "I think therefore I am", though that may in fact be the purest expression of solipsism ever.
The argument is simple. I know that I am aware and conscious. I have internal experiences, real feelings, emotions and desires and thoughts. But there is no way to tell if the people around me are experiencing the same things. They may tell me they are thinking and feeling, but I have no way to confirm that. They may be saying and doing all the things I expect people to say and do, but never actually be conscious, never actually feel or think anything at all.
Science illustrates this vividly. After all, we can dissect a person's brain, but we can't see consciousness, we can't determine whether there are actual thoughts or feelings there. No matter how far we unravel the circuitry of a person's brain, we'll never be able to discover if they actually feel anything.
The only way we can know consciousness is from the inside. And so we'll never know if anyone else has it. In the end, we might very well be alone.
This whole thought process comes from a mistaken distinction between being and experience. We assume that we can separate our experience of a person from the actual existence of that person.
To be fair, consciousness tends to support that illusion. After all, it erects a wall between us and the world, so that behind that wall, we may have thoughts and conversations with ourselves that no one else can see.
But this is a bit like being able to lift the hood of your own car, but being unable to lift the hood of anyone else's car. Just because you can't see anyone else's motor, doesn't mean they don't have one.
In fact, if you began to question whether or not all the cars around you had motors in them, we'd think you were crazy.
Of course all the cars have motors. We can tell there are motors because the cars are actually moving. We can see the exhaust, we can see them fueling up, we can watch the way they operate, and how fast they can go.
"But", you argue, "Maybe they just seem to have motors in them!"
Not possible. By definition, working cars have motors. The fact that they move means there is a motor. It might be a small motor, it might be an electric motor - but it's still a motor.
You might bring in an ultra-small car. "Look!" you say, "There's no room for a motor!"
It doesn't matter. I may not know where it is, or where it fits, but I know there's a motor. If it runs, it has a motor.
"Not this one!" you say. "This car doesn't have a motor! See, it just has this bucket of fuel, and that bucket drains into this little cup, and that cup periodically ignites and makes the whole thing move forward!"
That is a motor.
It may be a weird motor. You may be able to explain the whole process without ever using the word "motor". But it's a motor.
Now, admittedly, sometimes it's hard to tell if a car has a motor in it. A car might be moving so slowly, it's hard to tell if it's running at all. But if it is working, then it has a motor.
The same thing is true when it comes to human consciousness. If a human being acts like they are conscious, then they are conscious. I don't have to figure out where that consciousness comes from, how it works, or where it is "stored". I don't have to understand what's going on "behind the scenes". If it moves, it has a motor. If it communicates, it has consciousness.
This is exactly how we operate in normal life. None of us carry around a ghostbusters-like "consciousness detector", to pick out conscious beings from non-conscious beings. Instead, we simply interact - and in that interaction, we know that the being on the other end is like us.
There is no deep divide between experience and being. And so, we are not alone.