 He argues first that there is a one–to–one correspondence between simple ideas and simple impressions. He can’t prove that this correspondence holds universally, since he can’t examine every individual impression and idea. But he is so confident the correspondence holds that he challenges anyone who doubts it to produce an example of a simple impression without a corresponding simple idea, or a simple idea without a corresponding simple impression. Since he is certain they will fail, he concludes that there is a constant conjunction between simple impressions and simple ideas.