] Without a certainty about Jane's indifference, I doubt that Darcy could have persuaded Bingley to stay away from her. So Darcy persuaded himself that he was certain based on very little observation and even less knowledge. This is dishonesty, in what he tells himself if not in what he tells Bingley.

Darcy asserts in his letter that his conclusion that Jane was indifferent was not influenced by his desiring to find her indifferent:

That I was desirous of believing her indifferent is certain -- but I will venture to say that my investigations and decisions are not usually influenced by my hopes or fears. I did not believe her to be indifferent because I wished it; I believed it on impartial conviction, as truly as I wished it in reason.

However, I think it's himself that he's deceiving. Darcy is described elsewhere as an intelligent man, and yet I doubt that, if he could look at this situation objectively, he would not recognize that he had drawn this conclusion on very flimsey evidence -- observing a woman he does not know well for a short time in a public place. What signs of being in love with Bingley did he really expect to be able to see, especially considering the conventions of the time?

Darcy admits that he wanted to find Jane indifferent. The obvious motive is that he wanted to find her indifferent so he could "honestly" tell Bingley that she was indifferent. Two others that occur to me are that he wanted Bingley to marry his sister eventually, and that he did not want to be in a situation where he might be thrown togehter with Elizabeth, for example if he were a houseguest of the Bingleys, and Elizabeth was nearby at Longbourne and frequently stopping by to see her sister.