But some care is needed in using Descartes' argument. 'I think, therefore I am' says rather more than is strictly certain. It might seem as though we were quite sure of being the same person today as we were yesterday, and this is no doubt true in some sense.
[Problems of Philosophy - Bertrand Russell, Chapter II]
What does "says rather more than" mean? I don't understand that expression. Could you explain it to me?