Why are Sorrel, Longstreet and Lee hesitant to believe the spy's information?

The men are hesitant to believe the spy's story because they haven't had any word from their own scouts.

Sorrel said coldly, “Sir, don’t you think, if this man’s story was true, that we would have heard something ?” pg. 9

True, Longstreet thought. But there would have been something. Stuart? Longstreet said, “General Stuart’s cav- alry went out a few days back. He hasn’t reported any move- ment.” pg. 10