"Let us imagine the following case. I want to keep a diary about the recurrence of a certain sensation. To this end I associate it with the sign 'S' and write this sign in a calender for every day on which I have the sensation. — I will remark first of all that a definition of the sign cannot be formulated. — But still I can give myself a kind of ostensive definition. — How? Can I point to the sensation? Not in the ordinary sense. But I speak, or write the sign down, and at the same time I concentrate my attention on the sensation — and so, as it were, point to it inwardly. — But what is this ceremony for? for that is all it seems to be! A definition surely serves to establish the meaning of a sign. — Well, that is done precisely by the concentration of my attention; for in this way I impress on myself the connexion between the sign and the sensation. — But 'I impress it on myself' can only mean: this process brings it about that I remember the connexion right in the future. But in the present case I have no criterion for correctness. One would like to say: whatever is going to seem right to me is right. And that only means that here we can't talk about 'right'." — Ludwig Wittgenstein