Based on a series of quotations taken from the correspondence between the Bernese physiologist Albrecht von Haller and the Geneva naturalist Charles Bonnet, this article analyzes the project of the art of observation, one of the main themes running through Charles Bonnet's multidisciplinary work. It shows that the art of observation, as a theory of experimental methodology, was linked to psychology, focusing in particular on the capacity for attention. Behind attention, however, lies a metaphysical conception of freedom and will: it is because nature is the reflection of divine will that man can read and understand the book of nature.