How do you know anyone else has a mind at all?

You don't, not for certain. You only ever see what other people do and say. You never get inside their heads the way you're inside your own. That gap is called the problem of other minds. Your own thoughts are felt directly. Everyone else's mind has to be inferred from the outside.

The usual answer is an argument from analogy. You know pain makes you wince, because you've felt it from the inside. When someone else winces, you assume the same thing is going on in them. After all, their body and behavior look like yours. It's a reasonable inference, but it isn't proof. Similarity is good evidence, never a logical certainty.

In 1953 the philosopher Karl Popper suggested a more practical test. Ask whether you'd seriously argue with the thing. Arguing means crediting the other side with reasons and intentions. That already means treating it as a mind. As Popper put it, we do not argue with a thermometer.

Push the doubt all the way and you reach solipsism. It's the position that only your own mind is certain to exist. Bertrand Russell thought it was psychologically impossible to believe. He once got a letter from a logician who said she was a solipsist. She was surprised there were no others.

In daily life, nobody waits for proof. People have a theory of mind: the knack of crediting others with beliefs, desires and feelings. Children build it up in stages. Eventually they grasp that someone else can believe something false, or hide what they really feel.