You know what a headache feels like from the inside. But how can you know that another person feels one too?
A friend winces, rubs their temple, and asks for quiet. You have good reasons to think they are in pain. Yet you cannot step into their experience and inspect it directly. Could they be behaving exactly like you while feeling nothing at all? The thought is possible to imagine, even if it seems a strange way to live.
Philosophers call this the problem of other minds. Some appeal to similarities between our bodies and behavior; others point to the best explanation of what we observe, or to what people tell us. Some ask whether we perceive feelings directly in expression and action. There is no single agreed solution to every version of the puzzle, and everyday trust need not wait for one.