Phpunit Laravel Test Error

I have the following test:

class AuthTest extends TestCase
{
    use DatabaseTransactions;


    /** @test */
    public function a_user_may_login()
    {

        $this->login()->see('Welcome back!');
    }

    protected function login($user = null)
    {
        $user = $user ?: $this->factory->create('App\User', ['password' => 'password']);

        return $this->visit('login')
            ->type($user->email, 'email')
            ->type('password', 'password')
            ->press('Log in');
    }
}

But its throwing the following error: ErrorException: Undefined property: AuthTest::$factory

Any ideas why it's throwing this error?

via Chebli Mohamed