second thought

Used to indicate a change of mind after further consideration.

• I was going to buy the blue coat, but on second thoughts, the black one is more practical.

To begin to doubt a previous decision or opinion; to reconsider something.

• She began to have second thoughts about the deal just before signing the contract.

This describes doing something immediately, without pausing to consider whether it is the right thing to do.

• When she saw the shoes, she bought them without a second thought.