“But surely, I know what pain is & that I always have just that when I say ‘I have pain’”. Doesn't it strike you as odd that you should know so well what pain is, now when you haven't got it?! This rather suggests that you don't need to recognise any private object to know the meaning of pain. Nor can you say: to understand the word pain it's necessary to recognise pain when it does come. For who is to say whether you do recognise it, unless recognising here means feeling (uttering) recognition, not recognising rightly. In this sense I could be said to recognise Smith as being Jones.