Theory of mind postulates that, even though these cannot be directly observed, we readily impute mental states to others (and also to ourselves, since the bedrock proposal is that we understand our own mental states well enough to generalize from them). And so we constantly infer someone else's intentions, thoughts, knowledge, lack of knowledge, doubts, desires, beliefs, guesses, promises, preferences, purposes, and many, many more things in order to behave as social creatures in the world.