I was reading "Existentialism is a humanism" here. Though most of it is clear, I have trouble understanding how he makes the below claim which comes near the end of the lecture. How do you go from "I think" to discovering others?
...Contrary to the philosophy of Descartes, contrary to that of Kant, when we say “I think” we are attaining to ourselves in the presence of the other, and we are just as certain of the other as we are of ourselves. Thus the man who discovers himself directly in the cogito also discovers all the others, and discovers them as the condition of his own existence. He recognises that he cannot be anything (in the sense in which one says one is spiritual, or that one is wicked or jealous) unless others recognise him as such.