If we don't have Self knowledge how can we acquire knowledge of others thoughts.

 In philosophy, “self-knowledge” standardly refers to knowledge of one’s own sensations, thoughts, beliefs, and other mental states. 

At least since Descartes, most philosophers have believed that our knowledge of our own mental states differs markedly from our knowledge of the external world (where this includes our knowledge of others’ thoughts). 

But there is little agreement about what precisely distinguishes self-knowledge from knowledge in other realms. As a result there are competing accounts of how we acquire self knowledge.

Do you feel that you  The strongest epistemic claims on behalf of self-knowledge are infallibility and omniscience, by example 'I feel ill' I feel depressed' I feel angry'