The wish is father to the thought
Definition 1 of 1
Pro Tip 1/3Positive Bias
When people want something to be true, they may believe it without enough evidence. 
They presumed the weather would be sunny for the picnic because the wish is father to the thought.
Pro Tip 2/3Wishful Reasoning
This phrase warns about basing opinions on desires rather than reality. 
He's convinced he'll win the lottery—clearly, the wish is father to the thought.
Pro Tip 3/3Caution in Use
Using this phrase may suggest someone is not thinking logically but emotionally. 
She bought the expensive dress, thinking she'd find an occasion; the wish is father to the thought.