The magistrate appeared at first perfectly incredulous, but as I continued he became more attentive and interested;
"You may easily believe," said he, "how great was the difficulty to persuade my father that all necessary knowledge was not comprised in the noble art of book-keeping; and, indeed, I believe I left him incredulous to the last, for his constant answer to my unwearied entreaties was the same as that of the Dutch schoolmaster in The Vicar of Wakefield: ’I have ten thousand florins a year without Greek, I eat heartily without Greek.’
There are no more uses of "incredulous" in the book.
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I find it incredulous that you believe she is sincere.