You have direct proof of exactly one mind existing: your own

You can never observe another person's consciousness, only their behavior and a body like yours. John Stuart Mill argued this licenses an analogy: since others act as you do in similar situations, they probably feel as you do too. Critics note the entire inference rests on a sample size of one, making it the shakiest assumption most people rely on every day without noticing.

— John Stuart Mill, An Examination of Sir William Hamilton's Philosophy — 1865