At times like this he begins to doubt his own sanity.

As the deaths continue he starts to even doubt his own sanity.

He begins to doubt his own sanity and whether the adventure even happened.

There was no other alternative if he did not want to doubt his sanity.

"Never heard of her," said Harry, looking at me as if he doubted my sanity.

In such a situation the average person might start to doubt their sanity.

At the hospital Jennifer appears to him again, but he begins to doubt his sanity when she doesn't speak.

A person can come to doubt her sanity this way.

For the first time in six months, I began to truly doubt my sanity.

If such a move was supposed to make Buck doubt his own sanity, it hadn't worked.