"A physician's physiology has much the same relation to his power of healing as a cleric's divinity has to his power of influencing conduct"

This quote by Samuel Butler is suggesting that a physician's understanding of physiology is similar to a cleric's knowledge of divinity in terms of their power to influence the world. To put it simply, a physician's understanding of the human body and its functions is important to their ability to recover, just as a cleric's understanding of spiritual teachings is essential to their capability to affect people's habits. This contrast suggests that both professions need a deep understanding of their respective fields in order to succeed. It also suggests that both professions have the prospective to make a positive effect on the world. Eventually, this quote is a suggestion that understanding is power, and that those who are experienced in their field can utilize that understanding to make a difference.