At Johns Hopkins Hospital Craib evolved his fundamental theory of the doublet hypothesis which revolutionized electrocardiographic thinking. Until then, it was held that electricity in the heart differed from electricity elsewhere, in that it consisted of negativity only - a view known as the negativity hypothesis. He was to show that electrocardiography in the heart was the same as elsewhere, and consisted of both negativity and positivity. This challenged the views held by the establishment of his day and was met with reactive and, at times, destructive criticism.